There are plethora of solution you could use.
If you got $600 Bucks to burn, you can go with the professional solution: Adobe Photoshop. The standard in overly expensive products that do their job right. It's great for more advanced users, but beginners can get something out of browsing tutorials on the internet (Google is your friend here!)
If you want a free solution, you want the wonderful program called "The Gimp" which stands for "GNU Image Manipulation Program". It is free, awesome, and at the same caliber as Photoshop is, but without the price tag. Although it's equally as powerful, it's not so "Newbie Friendly". The control scheme is definitely a steep learning curve if you come from Photoshop like i did.
The GIMP is free, and multi platform, so when you don't have a Windoez based computer around, GIMP is what you want.
All in all, go with something like Photoshop if you have the cash, but give GIMP a try too. With a few tutorials, you will be making banners like the pros