The amount and condition of brake fluid in your car can affect how quickly your car stops and should be checked on a regular basis.

Locate the brake fluid reservoir beneath the hood first. It's usually located in the engine bay on the driver's side and will have a rubber cap with printing on top of it. Shapes vary, but It is usually bolted to a large round canister that's attached to the firewall. Check your vehicle's Owner's Manual if you are not sure where the brake cylinder is located.http://www.youtube.com/watch?v=-jU8JIfDwIU&feature=player_embedded 

1. Clean off the cap with a towel or rag before opening it. Do not allow dirt or dust to get inside the reservoir.
2. Slowly lift the cap without letting any brake fluid drip onto a painted surface; brake fluid is very corrosive and will cause damage to the exterior of the car.
3. Under the cap you will see designations for the minimum level and maximum level of brake fluid. You want the fluid to be at least at the minimum. Going over the maximum will cause the fluid to spill.
4. When you determine that your fluid is low, pour the brake fluid inside the reservoir to the maximum line. 
5. If you add too much fluid, extract the excess using a syringe or turkey baster.http://www.youtube.com/watch?v=-jU8JIfDwIU&feature=player_embedded