Going for 5x5 after doing nothing can make a person rather sore.
You are too upright in your squats. Stand up at tge top. Squeeze hard, break at the knees and hips simultaneously and immediately bend over and aim your chest at the ground. Your knees should not go much past your toes and the should get there by the time yoy are halfway diwn (use a tubow) and they stay there until you are halfway back up.