I'm 14 and I don't think I've worn a proper bra, I have a mum which I'm too embarrassed to ask and if I did, she'll probably make a joke or tell everybody. I went with her one time but I was really embarrassed and said no but I really wanted one, what do I do

First, its not a good idea to piggyback your question on someone else', you should start your own thread. So I've moved your note to its own thread.

Why do you think your mum would do that? Do you have a grandmother or aunt you can have talk to your mother. It sounds like she is will, so I don't understand your embarrassment.