Might I suggest you start with an "inexpensive" program to get your toes wet and decide whether you want to spend more money on a better program.
NO COST programs - The Gimp
http://www.gimp.org/ and Pixia
http://www.ab.wakwak.com/~knight/ Free, but not many tuts online.
LOW COST to NO COST - Adobe PhotoDeluxe. No longer being sold by Adobe. Most scanners came bundled with it. You might have one sitting in a drawer. Limited in what it can do, but it does support layers. Try looking on Ebay for a cheap buy if you don't have it. You can buy one for under $25.00 including shipping.
AFFORDABLE - PSP, PS Elements, PhotoImpact, DIP, etc. I'm a PSP user myself and am very happy with it. I also own PS Elements, but unistalled it.
EXPENSIVE - Photoshop, but who can afford it. $609.00 smackeroos. Great program if you half way know what you're doing and have money to burn.