I think 30kg is a good amount to start off with, because nomatter your starting point it shouldn't be too heavy, but it would be heavy enough for you to feel the burn. If you find them too hard to do at first, to start off you could hug a weight to your chest with your back against a wall, and focusing on your legs squat down and up using the wall to support yourself. You want to imagine that your feet are glued to the floor and you're pushing your body up against them - you want to focus on your legs and ass doing all the work. When I do them, I tense my abs and stick my butt out a bit so that it doesn't hurt my back. We squat 80kg at the moment, I get light headed after 4 * 10 reps of that sometimes, haha.