What are some good gym exercises for people with bad backs?

There are many exercises I am afraid to do because one small slip up can land me in bed for weeks sometimes months, and the last thing I need is more chronic pain, but weight gain will equal more chronic pain anyways so: I’m 23 5′1 28 % BMI I have no money for a personal trainer, Dr.s took all of that, but my bf is paying for a gym membership at a 24 hr gym just down the road from us and I would like to learn how to use it, but I can’t ask there because they want me to pay them more for extra help.

Example of bad back:

Sciatica, pain shoots down leg and is severe in the lower tailbone region on both sides, I have to be very careful when walking, standing, and shifting weight from my sides and I don’t get to sit still for more than 3 minutes. And due to this existing chronic pain for the last 5 years I now have muscle tension from hell all down the right side of my spine. The sciatica was caused by falling out of a tree and landing on my tailbone and the muscle tension issues from having to cope with the sciatica.

So what are some good back exercises that strengthen the muscles that probably won’t kill someone with my disabilities? Are squats even possible with my disabilities???

Oh, don’t tell me to go to a Dr. because this is what they tell me, ” We can’t help you, none of our information works, but we’ll take all your money and then some, so deal with it.” I paid a lot of money to get that information.

Anyways, thanks for your help, I need all I can get.