If you've got back pains (as I do from past sports injuries), I'd recommend just starting with generic resistance-training with weights to build strength in you back, core and hips. Focus on that for at least 6 months to build proper strength and support. Powerlifting is extremely physically-stressing and rigorous and if your body isn't ready for the demanding, complex and explosive movements you may land yourself an injury.
As for learning. If you don't want to/cant afford to pay for a personal trainer, you'll have to commit some time to your own research. You can find plenty of tutorial videos on youtube, or by following people on instagram.