As Slapshot indicates, professionals use Photoshop (as do I) but, at around £800 for the full version, it hardly meets your requirement for a free program!
Fortunately there's a free (open source) program which mimics Photoshop, called GIMP:
http://www.gimp.org/
Like Photoshop, there's quite a steep learning curve required to get the best from the software. (Working with layers can be rather tricky at first). If you install GIMP I strongly recommend downloading (and reading!) the documentation which goes with it.
However you might find it easier to view some video tutorials. Part 1: