For the wink issue effecting the lumbar there are a few key things to remember;
1. Start a squat from the hips first; the first axis to move is from the hips. If you move from the knees the exercise is injurious, nearly guaranteed.
2. Try to keep tension on the hamstrings the entire time, not intense tension but they are used like a spring throughout the workout. The workout utilizes nearly the entire posterior section of the body and portions of the anterior. Most people drop from the knees first and don't activate their hamstrings creating serious injuries if any weight over about `70 pounds is lifted.
So these 2 major concepts determine which of these 2 happens.
You don't need a trainer necessarily. Just a bar, some time and a mirror or a camera. Other things are debatable, some people use wedges under their feet or pads, belts, straps but as a beginner you will likely need none of those. Just good form with not much weight.