There's a lot of blank t-shirt companies and if you're really looking for a particular style you're gonna have to go out there and purchase a few to see which style would work best for your product.
In regards to software there's a bunch out there to purchase, however a lot depends upon your computer savvy and what type of application you are planning on using the software. For example as Lewis stated, Adobe CS3 is great, which I do use, but is not necessarily the most cost effective program if all you are using it for is t-shirt creation. If you go to
www.freeserifsoftware.com they provide either free, if for personal use, or extremely cheap if you want to use for a commercial purpose, a lot of the programs similar to Adobe, not only in use but also in functionality, and very easy to manipulate images, photos, etc. Another great website is
www.blender.org.
For cost effectiveness, I'd have to agree with Lewis again that screen printing is the least expensive of the printing methods at least when it comes to standard 6-color type prints and if done in high quantities. For more ornate graphics, CG images, etc.,
DTG or other transfer paper with ink is the better, if not only, method. The whole cost of production really depends upon your business plan and what market and/or market segment you are trying to sell to.
In need of advice 
