I HATE IT when clients use the leg press machine without knowing how to use it. Don't get me wrong, it isolates the quads, hamstrings, and gluteus but it can also wreck havoc on the knee joint capsule and the ligaments in the knee if not used properly. That means it can result in knee tendonitis and a whole host of other issues. If you don't know how to use the leg press machine-DON'T USE IT. My advice is perform squats and/or lunges to build strength in the lower body. Add weights if you want to add more intensity. It still works those same muscles but avoiding load compression related injuries.
*Image from cybex.com