As mentioned in the main post, one of your primary issues is going to be your flexibilty and mobility. To explain, this basically means that you need to pre-train your body to be able to achieve the correct positions you'll need in order to safely and effectively squat the weight without any inherent muscle tightness or immbolibility of joints causing you to put the strain of the movement on places which it shouldn't be.
The best methodology is to remain patient, I often bring people straight back to body weight squatting until you are 100% confident and balanced on your heels deep in the hole at the bottom position of the lift - to the point where you can just 'hang out' there. You want to have the flexbility and control so that you can hit your full range of motion with the bar on your back and no fear of inadvertently leaning forward.
This is one of the best stretches to help accomplish this:
You can additionally use a broomstick or dowel rod and practive 'body weight' overhead squatting to help achieve more full body mobility to prep you for the perfect squat. From perfecting your foundation, you're gains will skyrocket from squatting properly and recieving full benefits rather than feeling like you're going to die after ever rep ;)
(Just use a broomstick ;))