start up by setting yourself up with social media accounts where to display and share your creations (deviantart, tumblr, facebook, pinterest, twitter, instagram etc, it's also pretty easy nowadays to set them up so posting on one updates the others or at the very least there are shortcuts for quick sharing), focus on ones that target the kind of audience you want. If you haven't figured out your audience yet or just want all the audiences just go for every social media ever to be safe. It will take time and luck to build up a decent enough follower base. It's obvious maybe but having same username on all common platforms helps a LOT. Go check out the online presences of already popular artists for examples (loish, artgerm, alicexz, gingerhaze and such). Make sure you tick the boxes that enable google searches for you where applicable. Take your time and upload art at a steady pace to build up enough followers/watchers/likes.
You can set up shop on websites like Etsy (they charge a fee for listing items), Storenvy, Society6, Redbubble etc. or sell your own stuff either directly or for example via eBay. Some merch selling websites like Teefury and Teepublic specialise on t-shirts, some like Society6 let you print pretty much anything from shower curtains to wall clocks. Deviantart allows sale of prints and other merchandise from original art as well. Pretty much everything works on the basis that you get a set % or amount from the sales. Selling stuff directly yourself will cut out the middleman and enable you to keep a bigger % but will also mean a lot more work and planning as you will be solely responsible for the printing, selling, packaging and shipping.
If you do commissions for the love of god use Paypal (or cash obviously) and get paid before even starting work. There are complete guides written about doing commissions, most on deviantart. Have good dialogue with the customer and keep them updated, when you do commissions remember they are always right. Do not underprice yourself. Sit down and think how much work goes into a piece, if it takes you five hours to paint then at the very least charge local minimum wage and material costs for those five hours. None of that $2 commission crap, it's not worth your time unless you live in a country where $2 is an average daily wage. Rather just do completely free kiribans or limited requests if you need publicity or want to reward/surprise your watchers (which is also very important when marketing yourself online on social media, even a simple "thank you"-picture goes a long way). Free requests and kiribans are also a good way to gauge actual interest. If you can't give away your art for free you sure as hell won't be able to sell it. Set rules and limits, otherwise you will get mostly asked for furry porn. (Having said that, if furry porn is your thing there are whole websites entirely dedicated to it.) Do not use deviantart's own commission system because that will skim some of your money off the top and there is no need for that. Read other people's commission rules and guidelines for hints and tips. Always remember whatever you post online will never go away and someone will always take it and repost it somewhere else without your permission. Watermark/sign your shit if that kind of thing bothers you. In fact, watermark and sign your shit even if you think it doesn't bother you what happens to it, because if you don't you bet some asswipe will try claim it as theirs and sell it for money.
Commissions are only going to get you so much money and they are generally too infuriating for any artist to do full-time for a long period of time (vast majority of the people you deal with will be nice and civil but only takes one git that doesn't pay up or who changes their mind ten times to screw up your schedule for the month). Long-term other options that can happen is that a) you are good enough to be picked up by an established company based on the work you have been posting online b) you are good enough to be able to make a livelihood from just selling your own work in some shape or form, c) you are sort of okay and make a bit of money from selling artsies but ehh it'll always be a hobby or a part-time job on the side of a "real" job or d) you give up art altogether. There is also a rare and mystical option e) where you spot a job advertisement or hear word-of-mouth from friends in stable jobs about an opening, apply and get a job the old-fashioned way. There is also a spectrum of everything inbetween that can happen, there are no two art careers alike.
Fan art will always sell better/get more views than original character art UNTIL your original characters or original art has its own fanbase (in which case other people will start making fan art of your stuff and you can officially say you've made it). For a boost in watches/likes do a mash-up of fandoms. Seriously, anything mashed up with Star Wars, Doctor Who, Disney princesses, the Minions, Harry Potter, My Little Pony, Marvel/DC universe, Game of Thrones, art nouveau/Mucha yeah... off you go. Even the artists already popular and comfortable with their own art sometimes do fan art because it can always direct new watchers not familiar with their original work to their site. Nobody spontaneously searches for an original character or artist they never heard of, fan art is a gateway drug. Having said that, you can't forcibly use it for your advantage because it is painfully obvious when someone draws things they don't really care about. Ultimately always just do what you want to do and what you enjoy drawing. If you aren't doing fan art because it's hella fun then you shouldn't be doing it. Do not give a shit about what anyone else tells you to draw (within limits of law and common courtesy at least) and keep in mind you can never ever ever please everyone. There's this girl that goes by the online username Viria, she started on deviantArt posting tons of Harry Potter fan art and whatnot few years back (and became relatively internet famous for it) but like all teenagers she has grown and her interests have changed so now she posts some basketball and swimming anime stuff and oh my god nowadays I love and follow her tumblr for one reason only: the pissy whinging anonymous messages she gets about wah wah how she doesn't draw Harry Potter anymore wah wah followed by the brutal but absolutely honest way she deals with the critique. Magnificient.
Be warned if you choose to sell fan art products always remember you are in the gray area of copyright law. If you don't own e.g. Adventure Time or Hobbit then be prepared the companies that do own the rights can at any time issue a cease and desist on your sorry butt and have whatever you are trying to sell removed. Comic con circuit is stricter on this as large companies just like to swat many birds with one stone and send legal staff there to remove any unlicensed material and slap the offenders on the wrist. This is where mash ups come in handy as they are more in the gray area than direct fan arts. Should mention as well that selling/distributing your work solely online is a little bit different from the comic con approach. Idea in both is the same (get your art out there and make money off it) but comic cons are essentially more like ye olde market stalls. You have to be willing to be noticed and to sell yourself in person. I know artists who love it and are really good at it, others hate it and avoid it, you need a salesperson mentality that not everyone has. Comic cons also take more preparation and upfront financial investment. Your geographical location can be a major factor too wheter they are a viable avenue for you to market yourself. On the plus side they are awesome for meeting other artists, getting worthwhile feedback and *gasp* networking with any potential employers or publishers.
At the end of it all develop a deep loathing for reddit, 9gag, imgur and any other content-stealing scumbag website where morons don't source your work and some git called xxxxbeerpong6969xxxx (I'm sorry if that is someone's real username!!) reposted your shit and got half a million views while your own page has barely 10k. Spend far too much time emailing Etsy's legal department to get phone covers ripping off your drawings taken off the site.
That's all I can think of at the moment. Hope this helps ringthree's friend or someone else..