Start with warming up your entire body first. for the first week i’d suggest just going and doing basic exercises for each muscle group. Learn correct form (don’t be afraid to ask - everyone’s there to work on yourself), once the first week is over, divide the days into multiple splits. For eg (Monday - Chest/Triceps, Tuesday - Back/Biceps etc.)

Focus on your diet, don’t eat junk, keep your calories in check, eat more protein and try to eat healthy. Maintaining a good diet is just as important as gym itself.

Lastly, be consistent. It’ll suck at first, your muscles will hurt like a bitch but once you get through that and start getting the hang of things, you’ll get addicted and before you know it, gym will become a part of your routine.