1. Head over to the Gimp downloads page: http://tinyurl.com/5vxjfu5
2. Download and install gimp. There are versions for windows, Mac and linux so pick the right one.
You may want to view some online tutorials but its a pretty simple program to get the hang of if you just jump straight in. I will give you a brief rundown of the program below.
All the tools are located in whats called the "toolbox". You can hover your mouse over the different icons to see what the tool is. Below the toolbox you will see the options related to that tool.
Another window you will be using a lot is the layers window. here you can create a layer so that parts of your image are separate and can be edited later or moved on top of each other.
below are some shortcuts that will save you an enormous amount of time and that i personally use the most:
Copy: Ctrl-C
Paste: Ctrl-V
Paste a copied image to a new canvas the size of the image: Ctrl-Shift-V
Rectangle selection tool: R
Move tool: M
Paintbrush: P
Eraser: Shift-E
Select all: Ctrl-A
Select none: Shift-Ctrl-A
To make custom shortcuts see the tutorial I made for that HERE
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