Start with Adobe Illustrator (vector software) if you're new to graphic software. Photoshop (raster program, is less forgiving).
If you can buy an older version then just make sure it's compatible with your Mac since Macs are notoriously not backwards compatible. For PC you should be fine. I don't think you should wait for a new release of Photoshop but you might want to look into the new Adobe Creative Cloud system of leasing software. It might suit you or not.
I think that the majority of t-shirt designers with some years experience are using a vector program plus a raster program. For raster, there's pretty much Photoshop and a bunch of other crap. For vectors, there's room for a lot of debate about which program to use. Obviously, Adobe has the lions share, but a lot of people swear by CorelDraw. Corel also has Photo-paint, a Photoshop like application that I wouldn't smile and give to an enemy. LOL
Less experienced t-shirt designers focus on vectors and stay clear of Photoshop.
Then some of us add some one trick ponies to the mix. I use Serif DrawPlus strictly for drawing. It's a dirt cheap program with a 25 year history and some very powerful brush/drawing tools. That's all I use it for.
I also use a few other programs like ArtRage, Microsoft Expression Design, and Manga Studio. They all have something to offer, but none of them are the total solution offered by Adobe Illustrator and Photoshop. Manga Studio is a comic book software that happens to have excellent sketching tools and is capable of working with high resolutions and print quality.