What to do at a gym??

Ok, so I decided I'm gonna start going to the gym recently, but there's just one issue here. Despite being a fairly athletic person and being in sports for most of my life, I've literally never worked out in an actual gym alone before T-T

I literally don't know what to do or what machines there are. I know the treadmill, weights, bike machine thing, and that's about it. I do a lot of biking already and I REALLY need to start doing stuff with my arms because I feel like every workout I do focuses on legs (like biking and running and stuff).

What are some good machines to try out?? And equipment? Are there any norms or anything I should know about before I go? I don't wanna walk in looking like a lost child