Know what calculators your math class allows, and which ones are forbidden. The most commonly forbidden calculator is the TI-89, TI-89 Titanium, and any other brand that can solve indefinite integrals.
Decide if you want a graphing calculator or a non-graphing calculator. The most common graphing calculators are the TI-83, TI-84, 84 Plus, and 84 Plus Silver Edition. Unless you're a genius at math, these will be a lot more helpful than a regular one. If you're unsure as to whether a graphing calculator will be worth the price, you can always start with a cheap calculator and buy a graphing calculator later if you find you need it.
Find the best place to buy. If you buy at an online retailer, you'll probably pay a lot less than if you go to a store and buy it. For even greater savings, new, or only lightly used calculators can be found on eBay.