the most of us use photoshop you can find a trial version easy on the net but for full use you'll have to buy it but you also have the gimp, or paint shop pro, ....
you use photoshop which is a program that isnt free. You must buy it but there are other way to get it for free.
your welcome and have fun with photoshop try pixel2life.com for tuts or even here at GR in the tutorial section