If the front brakes have been squealing loudly, you may need pads only. If the car or brake pedal has been shaking when braking, you'll need to have the rotors resurfaced (called "turning"), or replace them.  If the car pulls to one side while braking, but stays straight otherwise, you may need calipers. This is a sign of uneven wear of your brake pads caused by uneven pressure in your brake lines. If the brakes have a grinding noise, this means rotors are done, spent, wasted, or whatever you want to call it, just replace them. There are two bolts that hold the caliper to the pad bracket, and two bolts that hold the pad bracket to the steering knuckle. You may need both SAE and Metric sizes of wrenches and sockets, as well as bleeder screw wrenches. Also, you may need a set of hex (ALLEN) or star (TORX) key wrenches or a hex or star bit socket set. Consider using line wrenches to remove calipers. Theses wrenches have a better bite and reduce the chances of rounding off hex nuts fitting on the hose end. You can always return what you don't use (keep your receipt and boxes and parts clean/undamaged). If you get caught without something while the car is apart, you may not have transportation to go buy anything.

Summary:
Consider the symptoms of the brakes; for example: Determine the parts and tools you'll need. Buy more parts than you think you'll need.