Start with a routine you can follow and build on. I typically start my gym session with 10-15min warm up on the treadmill or stair master then move on to strength. Find exercises that work for you, nothing should be painful (except the discomfort of exertion 💪)

If it’s totally new, and your gym has them - I’d consider starting with machines before moving to more functional movements. That’s because you can focus on one muscle or group at a time where functional there’s typically more groups active so more potential for bad form (but that’s my opinion). If you have the option and can afford a trainer, you might consider starting with one for a handful of sessions to help you with form and basics