In a general gym, first of all, when you join a club, a coach will give you a physical fitness test (there will be different tests according to the size of the gym)

Then there will usually be a coach to take you a little and teach you some basic movements so that you can use the fixed equipment freely. Because you didn't specify the purpose of your going to the gym, it's really hard to say exactly how to make a training plan. Make a training plan according to the specific data of physical fitness test and your needs.

If you haven't exercised for a long time, you should do recovery training first. It doesn't take too long, it doesn't need to be too heavy, and it doesn't need comparison. You can do it about three times a week. After practicing 1 month, the training mode can be adjusted appropriately.