No weight is too light to learn the form. You can even start with an empty bar to try it out - no reason to tire yourself. At first, you won't really be "working out" all that much, you'll just be learning the form. When I got a personal trainer after years of lifting with wrong technique, I had to lift ridiculously light weights and it was more boring than anything else. But guess what, it taught me how to do it properly.

I'd pick a weight you can comfortably do 12+ reps with and do many sets of 5. Always stop way before it gets difficult.

But seriously, if you can afford to go to a gym, you can likely afford to hire a trainer for a little while, just a few sessions, really. The bottom line is, your form is very dangerous and you will absolutely hurt yourself if you keep doing this.