You need an image editing program.
The very best is Adobe Photoshop CS4, but it is very expensive.
A more sensible choice might be Photoshop Elements (current version is 7), which is Photoshop's smaller cousin - can't do all the same things, but it is much cheaper and perfectly adequate for most people.
Another editing program you can get is the Gimp (free and legal). It isn't pretty, has a big learning curve, but is a great program. http://www.gimp.org/
Or you can go to these free websites, but that option is nowhere near as good as having the program on your computer: http://www.picnik.com or http://www.picture2life.com/ or http://fotoflexer.com/ or http://www.sumopaint.com/
The last one is your best bet for more complicated tasks.
You will need to do a lot of reading and learning - those skills don't happen without effort. Luckily, there are lots of resources available to help you. Start by reading the program's helpfiles, looking for online tutorials, and checking your library for books.