Silhouettes look great on t-shirts. I've done them in different colors, but for my boy, I prefer them in black. Make sure you use a 100% cotton t-shirt, without any ribbing or texture (I once made this mistake with a tank top and it made for a very messy silhouette). Once you have drawn your silhouette shape, these t-shirts are easy to make using a freezer paper transfer.
If you happen to be using Illustrator or another computer program, you can print your silhouette directly onto freezer paper if you have a standard inkjet printer. Cut your freezer paper down to 8.5x11, and feed into your printer just like you would any other paper (just make sure it's printing on the non-waxy side of the freezer paper). Then you just cut out the shape with an exacto knife, iron the waxy side onto your t-shirt, and paint with black fabric paint. I like to put another sheet of freezer paper inside the t-shirt in case the ink soaks through. For more detailed instructions, see here.
If you are drawing your silhouette on tracing paper, you can trace your paper cut out onto the freezer paper and proceed from there.