You can use the leg press in a number of ways to target different muscles. You can target the quads (rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius muscles) with a low close foot placement or you can target the glutes (the gluteus maximus, gluteus medius and gluteus minimus) and hamstrings with a wide foot placement on the pad.
Make sure you are firmly seated with your head back keeping the spine neutral. Do not lock your knees at any point. To start unlock by twisting outwards the bars on either side of the pad. Then hold the grip bars and slowly move the platform/weight downwards, keeping your lower back firmly against the back pad. Push back upwards not locking your knees.