I've done a good amount of commissions. Here's the hard truth for you- you are going to be competing with the entire world for commissions on those sites, and with no much of a resume or portfolio to show off or follower count...you are going to have a tough time getting work.
I would suggest...drawing a few pieces and putting them up for sale, people are looking to own traditional art by and large, and that's what you'll need to show you can do. You'll likely have better odds of selling a finished Wonder Woman, that people can see in it's finished glory, than having someone roll the dice on what you are able to do by request.
Also...most trading card companies ( topps for example) hire people to do sketch cards. They send you a bunch of blanks for say...some star wars set coming soon, you sketch on them, send them bck, they randomly insert them into packs. They don't pay a lot, but it's something. It looks good on your bio, and they let you keep a small percentage of the cards that you can then sell on your own.
anyways...hope some of that was worth the reading.