join a gym

Some friends of mine tell me not to join a gym because then, when i stop it, i will have health problems….
Is that rue? Pls someone answer me!

Last month i went to the doctor and she said to me that i had to do some exams first to see if everything was ok because i used to have some serious problems in the past….
Then she was going to give me the paper that the gym wanted to have, in order to let me join in. But i am not sure if my dad or mum will pay for it(i’m 14 years old) cause they never pay anything for me and they let me pay anything that has to do about me on my own….
I guess i have to find money to pay my exams and gym, too….
But how? I tried to convince my mom to have me take a job but she want listen to me….If you have any ideas, pls help me.
Thanks