What are the main topics a beginner need to study to be a good christian?
Is it important to know the catechisms (catholic, westminster, lutheran)?
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It's tricky because everyone starts in a different place.
Personally, I would recommend focusing on the New Testament to start, and getting good guides or commentaries. What guides or commentaries are 'good' for you or not will depend - you really have to try ones and see if any resonate with you (your spirit is saying 'yes!') and seem to move you forward.
I personally started with a focus on books on the Sermon on the Mount, which contains a lot of advice about what to do from Jesus, and moved outwards from there.
There are a huge number of denominations and different kinds of approaches to Christianity, with a range of views on all sorts of subjects. I personally don't stick with one tradition when reading - Catholic, Protestant, Orthodox, non-trinitarian - I find all of them interesting and having strong and weak points. In the end, your best guide to all this will be God's guidance through discernment.
Anyone's spiritual development is going to be an organic process - I am very skeptical of one-size-fits-all recommendations for study for a beginner. In a sense, everyone's a beginner. :)