It depends if you are going to be an independent trainer there or an employee.
I assume you mean as an independent trainer.
The main things you should clear out should be:
1) Fee to train: per session or a flat rate for unlimited training.
– I recommend you do flat rate as soon as possible. paying per sessions can be confusing . and misundestanding will come up creating friction between you and the gym.
2) Who collects the money from clients.
-You should do it your self. you will be in control, otherwise you will be vulnerable for . error on your pay.
3) Does the client need to be a member of the gym. Try to get them a lower price.
4) Where should your clients park. This will be an issue during peak times and if your client is not a member of the gym.
5) How long is this agreement for? m-m, 6 months, a year If you become successful and have lots of clients the gym might want to raise your fees.
6) How many people can you train at one time.
Those are the ones that usually trainers do not clear and become huge problems as times goes by.