You can find iron on transfers that work with most at home printers. Load it as you would load paper, use the enclosed software, and off you go. Takes maybe ten minutes to set up and in that time you can have your iron fired up and ready to go.
I worked for a t-shirt screen printing shop a few years back and I'll tell you what I did. As the manager of the store my job was to drum up new business. I spent half of every day knocking on doors. I went to all the local high schools, (they are a good place to start) they have lots of clubs and groups and each one has their own t-shirt they wear. I went to bars and clubs. I went to business's. I went every where I could think of. I brought in a lot of business and really enjoyed myself.
The best places to start out with are, high schools, lawn care, bars, and car washes. All of those places go through a lot of t-shirts. When you go in to any of these places, make sure you have the information as to the different quality of t-shirts and what the amount of colors will cost in a screen printing job. Remember that printing on a dark color is going to cost more because you will have to do an under coating of white.
Just make sure you know what you are talking about before you go in to a place of business. You will be amazed at how fast your business will grow once you get into a few places.