Summarize:

If your mom doesn't agree with you or tells you no, try not to argue, yell or whine. Instead, remain calm and keep your tone of voice friendly and understanding. Then ask your mom why now is not the right time. For example, "When do you think the right time is?" or "When did you get your first bra?" Do this if you are uncomfortable not wearing a bra, but your mom still says no. Suggest getting a training bra, sports bra or a camisole with a built-in bra to wear in the meantime. After wearing these for a few months, bring up the topic again. For example, "I have been wearing my training bra for six months. I think I am ready to wear a bra now. Do this if your mom simply will not listen or does not understand that a bra will help you feel more comfortable about your body. Talk about the issue with a trusted relative, counselor or teacher. They may be able to offer you some advice on how to approach the topic with your mom. You could also ask them to discuss the topic with your mom for you.
Remain calm. Suggest an alternative. Talk to another trusted adult.