• Learn the art print market: Understand trends and what's selling. Join groups, read online, learn about the industry you're joining.
• Build a body of work: A good number of images of consistent style; your voice and vision; high quality; uploaded correctly; saleable content (upload work people want to own and hang on their walls).
• Create an online presence: Use social media to share and promote your work after you have done the steps above. DO NOT promote an unfinished site or a weak looking body of work. People will write you off and never look again, even if you add tons of great images later on. The internet is fickle and demands interesting content or they walk away. First impressions matter. Don't promote using personal accounts, create business accounts on places like Facebook, Twitter, Pinterest, etc.
• Try to participate in local/physical shows/galleries/art fairs: This is good experience, and puts your work in a different context, giving you more opportunities to meet people and to present your work in unique ways both on and off line.
• Keep yourself in front of your buyers: Advertise to your audience, share interesting things with them, build a mailing list, interact and keep on interacting. You want your work in front of as many people as possible, but you don't want to be labeled a spammer. Fine line there, walk it.
• Be patient: It takes a while to do all this, years in most cases. Going full time with this is not a passive endeavor. It's full time work.