SQUATS!
This is me back in my first powerlifting competition squatting 180kg (400lbs) this is my favourite exercise because not only does it build strong legs but it builds a strong back and core.
Heres a few tips... for a low bar squat (what im using in the video below) typically you want have a nice wide stance with the toes pointing slightly out (this activates the glutes more) and as you drive up you want to drive the knees out aswell.
High bar squats - bar rests on the trap, and your stance should be about shoulder width, again with the toes slightly pointed out and drive the knees out as you drive up.
Check out the link the below and see if you can learn anything from it and comment if you have any questions.
Cheers!