single legs. they're really hard but if you stand in between 2 railings, you can use those to assist you on making the reps. the biggest thing with them, however, is staying on your heel. i learned by just starting in the crouched position, then pushing myself up. anotherwards, i just did the concentric phase first.
Stick to basics if you r not used to exercising your legs. Big muscle group and you need to be careful; you don't want to lose balance when performing exercises cause injuries r waiting for you at the corner. Go for squats (Smith's machine), leg press (horizontal and incline), leg curls and extensions and calf raises. Simple is good mate, simple is good....!