It doesn't take much to get started. Beyond the "equipment" you'll need - a clothes iron, and a printer - the material is inexpensive. One of the greatest bits about it is the fact that you can use any old shirt. In fact, you can easily refresh an old garment with a cheeky graphic, while having a bit of fun as you do it.
The easiest, and most accessible form of customizing clothing is as aforementioned - shirt transfers using a printer and clothes iron. All you'll need is a few transfer sheets, which can be had at a rate of 15 CAD for a set of 10 or so. Even cheaper in the U.S. of A..
Similarly, a printer and a clothes iron (which I would be confident in saying most people have), can be obtained from a thrift store for a total of $10 each. If you refer to my printer ink refilling tutorial, you can also see how the printing step can be made even cheaper.
Using blank shirts, either old ones of your own, dollar store purchases, or thrift store bargains, is the best way to do it. Personally, I'm a big fan of using bright colors and brand logos, as it can make for a very amusing shirt. In fact, below, is my most recent shirt - the iconic Pocky brand chocolate-covered pretzel stick box on a red shirt.
As I was running a bit low on colored inks, and since I figured that adding more bright colors to an already colorful shirt might overdo it, I opted to use a grayscale image, which I'd then cut up to make it a bit more interesting.