All you need to get started doing some origami is a square piece of paper.
To get a square piece of paper from any rectangle piece of paper, all you need to do is fold one corner, to meet the other side of the paper like this:
Once you have a square piece of paper, you are able to follow different patterns to create different paper origami creations. If you search google for origami instructions, you will be given hundreds of patterns.
Here are some ideas: