I have my own clothing line and I also silkscreen all my designs myself.
There are two programs you will need to run your business:
1) Adobe Photoshop CS2 (Photo Editing Program)
2) Adobe Illustrator (Vector Program)
I actually suggest also having:
3) Corel Draw 12 (Vector Program)
Basically what you will need to do is draw your design out by hand, edit it in Photoshop, then process it in Illustrator or Corel Draw.
What Illustrator and Corel draw do is turn your design into a "math equation" so you can resize it without losing quality.
I prefer Corel Draw because it comes with a Trace program that you can click and it will automatically turn your design into a vector "math equation".
You can visit my website to see some of my designs:
http://www.PoisonAppleShirts.com
Hope that helps!
~Mark Lim
Oh, I also created a website for people who want to start their own T-shirt business from home:
http://www.IndieElite.com