I've not read the Bible yet but I'm planning on purchasing the KJV Bible, once I've got the money. But I've came across a book called The Street Bible: would reading this first help me to understand KJV Bible? How different are these versions?
First of all, these two translations are extremely different. Here's Genesis 1:1 in The Street Bible:
First off, nothing. No light, no time, no substance, no matter. Second off, God starts it all off and WHAP! Stuff everywhere!
And in the KJV:
In the beginning God created the heaven and the earth.
There's obviously a significant difference between these two in both translation philosophy and style. The Street Bible is a paraphrase that attempts to use "the language of the modern urban reader." On the other hand, the KJV is a fairly literal translation that uses Elizabethan English, which can be practically incomprehensible to some modern-day readers.
That said, though the language is very different, The Street Bible tells the same stories as the KJV and other Bibles. Reading it will therefore increase your understanding of the Bible, and allow you to make connections to the stories in the KJV or other translations.
Ultimately, however, the best approach is to compare a variety of translations and choose the one that is most comfortable for you. BibleGateway.com does not include The Street Bible, but it does allow side-by-side comparison between dozens of popular bible translations, such as more readable ones like the NLT and NIV and more literal ones like the NKJV and ESV.
Before selecting a translation of the Bible for reading or study, it is helpful to understand the goals of the various translations available and how they relate to what you're trying to accomplish by reading the Bible.
The way this is commonly categorized is "word for word" translations vs. "thought for thought" translations. Word for word translations seek to provide the best translation possible, so that the English word chosen for the translation best represents the exact meaning of the word used in Hebrew or Greek. Thought for thought translations seek to capture the overall meaning of the text, so that the concept remains the same, even though a certain English phrase might better capture the original meaning for an intended reader. Usually the goal of thought for thought translations is to get the meaning of the text across using modern, English vocabulary.
Most translations are a mix of the two types, and some lean closer to one side. Here is a visual chart to help you see how some popular translations fall on the scale:
As you can see, translations such as The Message and the "Street Bible" you mentioned would fall all the way to the right in the "Thought for Thought" category. That's because while they are easy to read and are translated into very modern English vocabulary (and sometimes slang, in the case of the Street Bible), quite a bit of liberty is taken in order to promote readability over translation accuracy. The NASB, on the other hand, falls far left on the scale, choosing translation word-for-word accuracy over readability. All of these translations have intended audiences and goals, and that's OK, but it's important to know the difference between each.
In your case, if you have never read the Bible before and want a good starting place, the KJV is not the translation for you. As you can see in the chart, the KJV is an accurate translation, but since it was published in 1611, the English vocabulary used is outdated and can be confusing to many modern readers. This doesn't make it any less legitimate as a translation, but since the goal of Bible reading is often clarity and application, it can be tough to achieve that when you have to essentially translate the words again into modern language.
On the other hand, choosing something like the Street Bible is also not that great an option, because so much liberty has been taken in the translation to make it readable, a lot of the original meaning of the text may become lost, especially within more conceptually complex passages. Essentially you would be reading a man's word instead of God's word.
I would suggest starting with a translation that is closer to the word-for-word side, but with moderate readability. This will allow you to get an accurate translation, but in easy-to-understand modern English. I personally love the ESV translation, and it is freely available at http://www.esvbible.org/. It is easy to read, easy to understand, but accurate in its translation. It is also supported and taught by many well-respected Biblical scholars and teachers.
The NIV is another good choice, and is quite popular and available. As you can see in the chart, the NIV is a little closer to the thought-for-thought side of things. This will make it slightly easier to understand for beginners, but also provides a high level of translation accuracy. This translation is also widely supported and taught by many scholars and teachers all around the world.
I hope this clears things up for you and helps make your choice easier!