If you're starting GIMP or paint.net will both handle .tga files and are free. GIMP is more complex than Paint.net.
I prefer to use Paintshop pro most of the time, version XI has the colour changer that is extremely useful and can be purchased a little cheaper than the current version on ebay. I also like the way it can take a photo of a wagon or boxcar at an angle then resize it so its flat on.
Having said that GIMP is easier to use when lettering, it gives better control over the size of the letters.
Cheerio John