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Mouse 
If you plan on spending a lot of time on the computer, you want to find a mouse that's as ergonomic as possible. It should fit your hand comfortably and it's usually a good idea to get a mouse with a scroll wheel between the two main buttons as seen above. This allows you to scan entire web pages easily.
Keep in mind that it may be a good idea to buy the keyboard and mouse together in a bundle, but if the bundle doesn't have an optical mouse you may want to buy them separately.
What's an optical mouse?
Optical mice were introduced several years ago and use laser technology rather than the traditional mouse ball to track the movement of the mouse. This does two things, it increases pointer precision as well as keeps the mouse from getting dirty. A common problem with the mouse ball is that it can gum up and cause the pointer to stick or jump. I would highly recommend the optical mouse since they are common enough to not have to make a huge investment to get one.
Cost
Between $15 and $75 (Average Cost: $25). You will definitely want to go with an optical mouse as they are more precise and won't require any cleaning. It may be a good idea to get the keyboard and mouse together. Wireless combo sets will run about $50 - $150 .
Where to Buy
TigerDirect: Input Devices