For 2d art (digital paintings) you should maintain an online presence on something like deviant art, ArtStation, or beehance so that people can see your quality of work. On deviant art people can order prints of your work and you get a 20% cut. Alternatively you can direct people to a personal website or blog where you can sell prints at a much higher profit margin. Staying active in art forums is another way to get your name spread. The more recognition you can garner, the more people you have going to your website. I'd recommend maintaining at least one art showcase site, a tumblr blog, and a personal website, along with occasional participation in forums like cg society, polycount, or relevant subreddits. The important thing is that you are doing lots of art and showing it to lots of people. You might even be able to have a youtube channel or a twitch stream (which you can also monetize) where you can stream speed paintings. Really anything to self-promote.
This self-promotion is really important because prints are only a tiny fraction of what you'll be able to make doing digital art, your main income will come from commissions and freelance work; and that doesn't happen unless people can see you being active (and skilled). Advertise that you will do commissions on your website and art-related social media, but also seek out freelance jobs that fit your skillset (there are lots of websites dedicated to helping freelancers find freelance work).
Also, selling prints or originals is a lot easier when you are in person. Going to things like anime conventions and selling fan-art is a great way to spread your name around. Unless you're really good, and depending on the conventions you attend, you may be lucky just to break even money wise, but getting people to look you up and selling those commissions is your real goal (just make sure you're not sinking yourself too far in the hole).
Finally, there's the patreon option. If you can maintain it, patreon's are great for a little bit of consistent financial padding, but unless you're lucky, it's probably not going to pay the bills by itself.
My final piece of advice is to not undersell yourself. Always make sure that any commissions you do are taking in to account materials and labor. Time yourself. Do not under any circumstances work for less than minimum wage, too many new artists are eager to please their customers by offering the lowest price they can, but that is no way to put food on the table (and frankly just exacerbates the issue of people not properly valuing artist's time). Along this line of thought, don't ever work without a contract either. Always have a written agreement about payment and time frames signed between both parties before you even start sketching. You don't want people wasting your time, and you definitely don't want people trying to weasel out of paying you.
To sum it up. Being a full time artist is as much about being a marketer as it is being a creator. And if you do enough of it you might just be able to quit your day job.