Well, for the forum part you have the basics (ask questions and make comments)

The content part (skins/ mob skins / dat- & texturepacks) depends on whether you are:
  only 'shopping' (in which case you can look through te sections and download things [​although it is considered good form to comment and/or diamond {clcik on the diamond icon} things you like or download]),
    or
  also produce such material — in which case you have the added oppurtunity to see others comment on and potentially use your creations. [Instructions for uploading are generally at the bottom of teh browsing page as here .]

More specifics can wait until you've had a chance to poke about and refine your questions.

I would need you to be a bit more specific to be able to help much, I'm afraid.

oh hi there cutie