There are couple freeware softwares I could recommend.
http://www.getpaint.net/
http://www.gimp.org/
There are punch of plugins for both of those apps. Just google their name and plugin you're looking and you should be find those fairly easy.
One thing you need to know about .tga files is to learn how to use Alpha Layers. This is a layer where where the transparency information is stored. There's plenty of good guides about Alpha Layers out there.
