If you can afford a trainer, I'd get one. I have bad knees too and I really regret not getting one earlier on. Maybe I could've avoided some of the problems I'm going through now, I'm just not sure?
FWIW, I'm a former athlete, I've lifted/exercises most of my life and I still must have been doing something wrong.
Obviously, it's possible to exercise without a trainer but if you can afford one I'd definitely go for it. Especially because it can help a lot with motivation!!
Good luck with whatever you decide.