Many artists such as myself have a hard time figuring out how to promote their artwork and sell it. The term "starving artist" is no joke. Thanks to the internet, promoting your artwork and designs is made easier for "us". There is a downside, however; it's not going to be recognized right away because you are absolutely throwing your images into an intense ocean of other images that just blend in with everything else. You need to market your work but that is a whole different topic I'll save for later.
Recently, I've stumbled upon Cafe Press. For a long time, I've always wanted to place a lot of my images and designs on T-shirts or other apparel. So I jumped right in and read their guidelines about how to prepare your artwork before uploading them. It's also a good idea to research their forum and read the tips on how to make your shop a "success". I am sure there are other sites like Cafe Press; but this is the one I use.
Before you start uploading images on their site, it's best to prepare your images so they look good in print. I personally use GIMP because it's a free, downloadable software for image manipulation. I like to make all of my designs transparent so that the white space does not show up on black / dark apparel. You also have to zoom in quite a bit to get every little 'speck' off your image and around the image. When you are finished, save it as a PNG file and upload it to Cafe Press. (I would not suggest placing your design on every product available.)
Now that you have your designs prepared, you can start putting them on T-shirts. Be sure the designs are something people might want to purchase. For example, I am drawing and painting all of my designs myself and even tweaking some of them to my liking. I also explain how I do everything and even scan rough drafts of my work in between. These are simple one-graphic sketches I start off with and then I add the color, and use a black uni ball pen to outline and make it look "pretty". However you want to draw and paint is up to you. I like the freedom involved with art.
You always have to start somewhere and this is currently what I am working on myself.