If you want to sell anything you should understand that in order to sell, people have to want. Learn how to make people want to buy your art. (this is called Marketing in the business world). One part of creating your marketing plan is creating and defining your identity a.k.a. your "Brand". It's what people will know you as and for. Next is promoting. You have to make yourself known, otherwise no one will know you! Social media makes that an easy job, but it takes a lot of effort because not everyone will be interested. I'd say right now get a lot of followers who like your artwork online or even offline then from there. Your focus then should not be making money, but promoting your brand, aka identity and letting people know who you are as an artist. When you've done all that and you have a solid following you can start letting them know your art is for sale. This isn't an etched in stone guide, you could start selling right now and make cash, it's just a matter of making it a long term business. Now how to actually sell the art? Depends if it is digital file or a physical painting. If it's physical, you should have them send payments through like paypal or something then you ship the shit to their address. If it's digital artwork then you do the same thing but you send them an email of the art or something. They have sites online where you can sell digital artwork.