If you want to advertise for free I would set up an Instagram account and start posting your art with some frequency, there is a metric for getting followers usually by following them. My art isnt advanced enough to sell really so I haven’t bothered but if I ever got serious that’s what I would do. One of the nice things about fine art America is you can sell the original and if you pay them some fee you can actually set up your own web page, one of the dangers of linking to a big site is people might see other art they like more and buy it instead so it’s good to have your own site. So basically my advice is to use social media to your advantage post your art on all the major sites, I follow a few artists even on Facebook and have bought prints and one original.