Welcome!
Start with your body weight exercises. Lady push ups, then normal push ups. Forward/backward lunges, for legs. Keep back straight, do not bend it forward, and do not move front knee off toes. Maybe two small dumbbells in your hands just to have better posture. If that sounds complicated - ask for help - one hour per week with personal trainer can help a lot. Plank is other exercise I would propose. Try to start with something called full body workout. That mean - every time all muscles are trained. Put 'Full body sample workout for girl" into google. For me that was one of the first results. Read, try if it works for you. If not - just ask another question. WE will be more then happy to answer.
There are two main points, or tips here. Changing body is long process, it takes months to have results. You are young, so it can faster, but still - all is about small steps. Every week worked counts, and next January, someone else will start. And then you will see how much you have done. Next tip - it is quite easy to get injured. Thats why I'm insisting on small steps. Watch your body, if something is painful - understand why, and what to do with it. Trainer can help, but at the end it is you, and your body, so be sure you understand each other. ;)
I assume that your back pain is in the place where bar was... if so then you can roll towel over the bar. There are foams to put on the bar as well. You body was not prepared for that, tissues got squeezed, so there is pain, perhaps bruises as well. With time, it will be better, after 2-3 months you will do it without a problem. Squat is complex exercise, so I would advise to ask trainer, or someone to watch, and tell you if everything is OK. Trainers are there, so you can talk with them. From time to time, they can look and give you a hint. If that is too often, or that person see much work - paid training will be proposed.
To select best trainer - talk with some of them, talk with other people at the gym - if you like trainer, and other people says his is known to be good. At least that is not random choice.