If you don't want to go through the hassle of printing the transfer and ironing it on a shirt yourself, there's a website,
www.cafepress.com, where you create you're own little "store" with as many designs as you want to be printed on over 50 different items. It's mainly used by fund-raising groups, charities, and family reunions, but it's a blast...if you use it, enter my site name as the person who referred you, fosterheritage, and I'll get credit for the referral (no, it doesn't end up costing you more).
All you have to do is create a design using either Word, Powerpoint, or any no. of other applications, save it as either a jpeg or gif file and download the design to your store. After that, select the article you want to place a design on, attach the design and it will show you what it will look like.
The cost is a little bit more than doing everything yourself, but there's a trade-off in only having to worry about designing what you want. If you're just interested in t-shirts, they used to use only heavy-duty Hanes which are pretty sturdy, but don't know what they're using these days.
Have fun!
-Richard