The first thing most will think of is photoshop. It's excellent, but expensive. And if you want to download a trial it's hundreds of megabytes. Personally I use GIMP (GNU image manipulation program). It is excellent; lightweight (under 20 megabytes), will run on many operating systems, has many useful tools, is very user friendly and has heaps of included gradients, fonts and textures!
Where can I get GIMP?
Installers are available for linux/unix, windows and mac operating systems. The system requirements are fairly low, and it will download quickly with an average internet connection.
I'm going to take you through the ABC of editing an image. If you want to learn to do something that's been missed, check the tool guide at the end or just ask!