Welcome! I think the others have said it best already, but here's a handy list:
1) From the drop down menu at the top, click Collection, then click Add To. Find the sport you want, the year, the brand (inserts are under the main brand) and start adding cards to your Wantlist and your For Sale Trade lists.
2) On the same Collection page, scroll down and you'll see a link called Trade Matching. Click this (once a week only) and it will generate a report based on what you have vs. what you want.
3) Click on another person's name in this report. Click Propose Transaction and select the cards you want vs what you want to trade, then click Submit, and go from there! Make sure you have some envelopes, thin cardboard and 9-card plastic dividers on hand; if you plan to ship a lot, some hard case protectors, toploaders and foam is good to have around as well. I didn't know this at first, and shipped my first few trades in sandwich bags taped shut, which was downright risky.
Hope this helps! If you have any Topps 2010 or Score 94 baseball, look me up!