You need to know why you chose the belief. What reason did you have? Make sure it is a good reason so you aren’t caught off guard or look bad.
Do plenty of research on it. If it is a different religion, look up the religion in textbooks or on the internet. If it is a style, you may do research if you want, but also you need to stand as to what you wear, and explain that it’s not evil, it’s just different.
On a piece of paper, record all of the data you’ve uncovered.
Confront the person you are trying to convince. If they often come to you, be prepared. Maybe practice discussing it with yourself, a friend, or somebody you can trust.
Firmly tell them that your religion/belief is what you believe in. Stand up for it, and don’t take their discouraging words. Have confidence that your belief is the right thing for you, because to the best of your knowledge, it is true.
Don’t get offended if others question your beliefs. If you are truly secure about them, then you have nothing to worry about.
Understand that some beliefs have no foundation, and need to be put out to pasture.
If your belief is controversial, don’t flaunt it.
Don’t wear your heart on your sleeve. Sometimes it’s best to just keep your ideas to yourself to avoid fights.