This might not be as easy as it sounds. Some brake fluid leaks can be downright hard to find. Start with a good inspection of all the brake lines and fittings. If you don't see any leaks there, remove the wheels and inspect the brakes themselves. Pay particular attention to the wheel cylinders. Pull back the dust boots on the wheel cylinders and look for fluid leakage. If you see any, replace the offending wheel cylinder(s). Lastly, check the master cylinder by holding your foot on the brake pedal. Don't push too hard; it's easier to diagnose with just a moderate amount of pressure. If the pedal slowly sinks to the floor, then you likely have an internal leak inside the master cylinder.
Be through in your search for leaks. Any leaks in the brake system are a problem and should be addressed immediately.