Photoshop is undoubtedly the best programme, but pricey with it. Its not too hard to get into if you get a "Class Room In A Book" guide, which are a pretty comprehensive start to learning the Adobe programmes (usually fairly cheap on Amazon second hand). Also when you feel you know whats what, there are thousands of free tutorials online.
A cheaper alternative is the Photoshop Elements series, which are basically a stripped back "home" editions of Photoshop, which contain most features but leave out more complex and powerful tools. Although one of my friends described it as "for grandma", it is a nice cheap way to get into Photoshop. Also by Adobe is Lightroom. I have no experience of it at all, but from what I hear it works really well with simple retouching and colour edits. You can download free 30 day trials of all of these from the Adobe website, so give it a try and see which you like.
Further afield, GIMP is a free Photoshop-style clone (downloadable from
www.gimp.org/), which has almost all of the features of an old edition of Photoshop. Its less stable, sometimes stiflingly slow, but ITS FREE!! I've gotten some pretty high end results out of it, but by God, it can get on your nerves with it's snail pace.
So to conclude, I would go Photoshop. Stick with it for a couple of months, get the book, read tutorials online and you'll get into it in no time.
I have it and use it sometimes. 
