A few years ago, I was in a similar situation as you; new on the site, most of my collection entered, some basic needs/wants identified, etc. I realized also that I needed to have some skin in the game, something worth offering in exchange for my wants. So you start with what you have.
Easy #1a: Check your collection for duplicate cards. Do you need more than one of any of these? (Some, sure! All, probably not.) Sort through and pull out duplicates as traders.
Easy #1b: One of the tools in each collection is an option to "Move all duplicates to For Sale/Trade list." This both helps you find the dups by highlighting them. Then you can go through and pull them, or at least drop them in a penny sleeve for later. Then if it's a card you really need more than copy, you can move the individual card back to the collection. (Don't worry that you have 2 or 3 stacks of the same card in your collection, there's also a function to bundle cards back into a single stack.)
Easy #2: Check your collection for cards or sets that seemed a good idea at the time but now aren't interesting or realistic.
Easy #3: Buy the occasional repack box from stores. Whether it's a couple random packs or a mixed box of cards, repacks are a pretty economical way to pick up trade fodder from different eras. Truthfully, there's a lot of the junk wax era and an increasing amount of current era. But I've picked up a bunch of traders from these boxes, particularly oddball cards and things that I'd never pick up on my own (read Bowman anything).
My hardest decision was putting a bunch of my vintage cards on trade, but I could also see that they were in demand and could be used to fill my own vintage and other goals.
The other tip I have is to take the time to view people's profiles and get a feel for different people's trading approaches. For instance, some will trade only the same era for the same era; others may be more flexible. Aside from helping you determine the right approach for trades, you'll get an idea for how you can make trading work for both parties.
Best of luck!
Collecting White Sox, Royals, and 1978 Topps Baseball. Trying to build a multi-sport HOF frankenset.
Done is better than perfect.