Yeah like Swaggin said, just start with body weight or a barbell and practice lowering slowly and keeping your back straight. I picture it like an accordion getting crumpled, bringing my chest towards my knees. About keeping your knees behind your toes, it's just a guideline. Physiology will push your knees forward if you're doing a full squat. If you try and keep them back, you either won't be able to continue descending or you'll fall backwards. For the squat and deadlift, it's vital that you use proper weight and good form. Also try and get at least parallel so you get the most benefits from the exercise. I see lots of people loading on the weights and doing little dips that aren't even quarter squats
And 5'9" and getting rim.
Maybe I'll be there some day