First, learn to set up your spine and shoulder girdle as described in the "How to Squat" section. Suffice to say, you should pull your shoulders back and keep them there. Additionally, you should NEVER let your back hump over at any time during the lift. Load a barbell on the floor to the desired poundage. Make sure the area around you is free of potential troubles and the floor is not slippery. If you don't have the required strength to use 45 lb. plates on each side of the barbell, elevate the bar to mimic the height as if you were using the 45s.