Well, I am just going to reply here as this is something that really should be stated for future readers.
As far as designing your own shirts using photos and stuff like that, you would really need to get a copy of Photoshop and Illustrator. The techniques for doing what you will probably want are pretty daunting in the terms of experience you need to pull it off well. This involves things like clipping paths, level adjustment, halftone screens, vector drawing and so on. To start with no experience (as I am assuming), it would be really difficult and take far more time to teach you than to get an artist to do the design for you.
However, if you are determined to do this yourself there are some simple tutorials around the web that will teach you the main things you will need to know. You can look around for tutorials on how to remove backgrounds from photos using Photoshop. That will get you a lot of the way there with what you want. It is pretty basic, and it uses the polygonal lasso tool and the eraser for the fine edges. From there, you would use the type tool to put in the text that you want, re-size and edit. Of course, this is the most basic of things, but it is a good start.
If you are just putting things together to give to your artist as a base idea, then Photoshop will work for you. You can collage things together and get a rough draft for someone to go off of. That really doesnt take any previous knowledge to do, but the more you practice, the better you will get.
I hope that gives you a base understanding of what goes in to what you are looking for.