Approaching this from a Catholic perspective, what are good signs that God is calling you to the vocation of priesthood, in other words, what must one look for to know whether you are called to the priesthood?
There is a book called, To Save a Thousand Souls by Fr. Brett Brannen, which deals with this quite extensively. It's an almost must read to get insight into the process and lifestyle.
My own pastor has shared stories that he knew at a young age that he was called to the priesthood. As a young boy of 8 or 9 he says he "just knew" which is in keeping with a seed planted by God for the vocation. Oftentimes, young boys who are part of church-going families will mimic the parts of Mass and that is something that shows their interest. Of course discernment is an on-going process and once such a boy becomes a teen, it is best for him to be able to further his discernment through connections with a seminary professor etc. In the meantime it is always important for Catholic families to help their children know about consecrated vocations. They can do this through wonderful books and positive conversations meant to show all the ways that God might be calling.
Other times a young man or grown man will feel a tug at his heart to become a priest. Again it seems to be God initiating the process but it is important for all people to be open to "hearing" God which is why studying Scripture and spending time with at least some like-minded friends will help each of us be able to see and follow the movements of God.
There was a time when discernment wasn't really part of the process to become a priest--when just about every Catholic family had a priest from its members; but times have changed and it is being seen more as a "call" and the depths of the vocation are better understood for today's men pursuing the priesthood.
First of all, it is important to be open. Many people do not even consider the priesthood. Next, God will not call you to be a priest if you will absolutely hate it. Doing His will will make you happy. Also, you must consider your age. If you are 15, don't expect God to tell you what you are going to do with you life just yet; you don't need to know yet. Just do His will in small ways now, and then you will be ready later. If you think you might be called to the priesthood, talk to your priest, and he will know what you should do next.
In my short months of trying to find God, I've read many books. One such that I am reading now is called "The Elijah Task" by John Loren & Paula Sandford.
However, before you go on to read this, please know that what I write is my knowledge from books from servants of God (which I digest and actually remember) and all of which are charismatic so, there may not always be a source from the internet and it might conflict with the teachings of your church. All verses quoted are in KJV only.
A major important biblical truth about the five ways God speaks to man; dreams, visions, dark speech, direct speech and audible speech (including callings, prophesies, revelations etc) is that if it is of God, He will make it happen and if He really called you to something (whether it is a place or to counsel or to leave pastoral and go to the mission field, even if it's just to pray for a stranger etc) He will provide the confirmation, training and means necessary. The bottom-line is this - focus on God, not the calling. Focus on the Giver and not the gifts. Focus on the Lord, thy God and seek first the kingdom of God and all these things shall be added to you.
12 And Miriam and Aaron spake against Moses because of the Ethiopian woman whom he had married: for he had married an Ethiopian woman. 2 And they said, Hath the Lord indeed spoken only by Moses? hath he not spoken also by us? And the Lord heard it. 3 (Now the man Moses was very meek, above all the men which were upon the face of the earth.) 4 And the Lord spake suddenly unto Moses, and unto Aaron, and unto Miriam, Come out ye three unto the tabernacle of the congregation. And they three came out. 5 And the Lord came down in the pillar of the cloud, and stood in the door of the tabernacle, and called Aaron and Miriam: and they both came forth. 6 And he said, Hear now my words: If there be a prophet among you, I the Lord will make myself known unto him in a vision, and will speak unto him in a dream. 7 My servant Moses is not so, who is faithful in all mine house. 8 With him will I speak mouth to mouth, even apparently, and not in dark speeches; and the similitude of the Lord shall he behold: wherefore then were ye not afraid to speak against my servant Moses?
From the book,
We must test our listening repeaatedly. God will never speak falsely, but, because of who we are, we may hear wrongly. As soon as we begin to converse truly with God, both our own flesh and Satan become upset.
Sometimes, and most likely in the case of the prophetic office, the vision does not happen and it causes turmoil or even persecution from their own brethren. However, this would not make the speaker a false prophet because yes there was a vision and yes it was confirmed to him by the Lord and by others. But, why then didn't the vision come true?
9 Having made known unto us the mystery of his will, according to his good pleasure which he hath purposed in himself:
The vision (or any other five ways God speaks) may have been the Lord's tool to humble someone or to teach someone something totally different from the vision. It is by His purpose that His will is fulfilled, not ours and not the pastor's and not the apostle's but His. What then, becomes of the speaker?
15 He shall call upon me, and I will answer him: I will be with him in trouble; I will deliver him, and honour him.
The book on this verse,
If confirmation comes and you are convinced of your vision, obey it no matter what (Acts 26:19; Deut. 23:21). You may be wrong, but the Lord will honor your intention to obey, He will forgive you, and He will haul you out of trouble.
It is difficult to know whether something is of God or not, agreed? Take counsel upon this verse,
2 Corinthians 13:1
13 This is the third time I am coming to you. In the mouth of two or three witnesses shall every word be established.
Take heed that the Lord will give you confirmations from total strangers or people who do not know what their words mean to you. However, you need to be dead enough to self to let that happen. I mean, is the words of a close friend easier to hear and internalise or the words of some stranger who came to your church for a few days? Or whom you don't even know the name of? If you are convinced of your calling, flow in the spirit and go about your daily life. But, be more aware of your surroundings and of the people you speak with. Self-awareness might also be the Lord's purpose in your current situation!
The truth is, we will never know the mystery of His will until He reveals it to us in His timing.
Most of the time, the Lord calls us to a work that we do not like or do not feel comfortable in. However, we must know that God is able to use the weak to glorify His strength and to use our discomfort and embarrassment in the service of others so that His glory may be shown through us. God wants us to be joyful in Him, yes. But, only He knows how we can be truly joyful. Merlin Carothers is another recommended author from me to you, read some of his books! I have read where God asked a family to leave pastoral, uproot their live into a whole different city, without knowing what's going to happen next! God knows everything and He knows how we can truly be happy, even if we feel discomfort or doubt in the first few moments.
Seek God and His righteousness first, focus on the Giver.
Like Charles said you have to ask someone else in the Catholic church to validate your calling (a bishop). However the rest of us sense the leading of the Holy Spirit and respond independent of any organisation or anything.
The easiest way to do this is try teaching and preaching the Bible if either are easy for you then you might and I emphasize might be called a teacher or preacher like for sunday school. You would have to learn proper soteriology.
That would not be the calling of priesthood which is something like what we call shepherds or as the modern church says pastors. For that you need to pas the test presented by ecclesiology ie the service of the church to the lay person. (The also involves other things but you pick them up along the way.) Then pastors need to be able to interact with people on a personal level and help them through the trials of life loving them just like they were your own children. I think the quickest way to discern if you have that kind of love is again to try it by interacting in a leadership role for children and adult at church as allowed before you are in training for the priesthood. ie you clean dirty diapers in the nursery, clean puke and discern lies in the children's church, pre-teens and teens present their own complex set of problems, mostly its counseling young people about moral attitudes the love walk and avoiding fights, booze drugs, porn and pregnancy. Finally you should examine the ministry to adults. Adults have the same problems as teens only they get into debt and have to deal with marriage and divorce. If you can survive that and still love God's people and want to do it all over again next sunday then sure yeah you're called without a doubt.
After you know you called and along the way you want to pick up some pneumatology the study of the spiritual realm. You're nothing without the Holy Spirit. I'd recommend becoming pentecostal (we have more fun) as it is allowed by the RCC.
Everyone is going to ask you about eschatology as its the lay persons favorite topic since the Left Behind series and especially since they are going to make a new and better movie at some point.
Finally learn some clinical psychology so you can give scientifically and biblically valid advice.
You're certainly going to make noncritical mistakes do your best to forget them and move on with life.