I'm going to confess to my parents
What is it that you want to confess to them?
I've decided I want to out my brother so they know what he is doing.
If you have done something that you need to confess to your parents, it is a good idea to own up and be honest with them. If it is your brother's secret you are going to tell, you should consider letting him tell himself instead of doing it for him; you might break his trust in you.
They need to know that he is gay.
They can find out eventually but you shouldn't be the one to tell them the secret. It is good to be honest with your parents, but with your own secrets, not with others.