There really is no "MAIN" forum.
Look up towards the top of your screen and you'll see a little box entitled "FORUMS".
Click on that and it will take you to a page that breaks all the different forums down. Pick the one that fits your question and click on that.
After you've snooped around awhile you'll customize the forums you look at in the "MY FORUMS" area. (little circle near the top right hand side of your screen)
In "my forums" you can pick and choose the forums you view.

After I joined it took a couple of days of just clickin on stuff to get the feel of how this site works.
Don't be afraid to just click on stuff just to see what it does, nothing will explode!