Well, you need to start somewhere. I suggest visiting the wiki at http://www.wiki.frictionalgames.com
I also have quite a few tutorial videos on my Youtube channel if you wish to check them out. I'm not too much of a fan of self-promotion, but I do like to help people out if I can >3<
You can try looking at this for a start. If you want to have some files to start off with, I got them too at this download link: http://www.mediafire.com/?m0yw9ro94yruzk6
Other than that it's mostly just work, work, work. The more you work with it, the more custom you become and it gets easier ^^
Ask questions, use the main Amnesia files for reference and Google things or search Youtube/the wiki.