First and foremost, welcome to the site.
There are a couple of different ways that you can sell on TCDB. For example, you can post cards for sale in threads (e.g. links to eBay sales, general sales posts, etc.) Take a look in the Buy/Sell/Trade forum for some examples.
However, as a starting place, I would suggest that you take the time to build up your inventory listing in your TCDb "For Sale/Trade" list in the "Collection" feature of the site. From there, other users can see what you have and match to their "wantlists" and send you proposals.
There are a number of helpful resources that provide step-by-step tuitorials regarding how to get started with the site. (Lots of helpful tips in the forums if you search them.) Here are a couple additional resources as a starting place:
Again, welcome to the site!