Is it a bad idea to get braces while you are missing a tooth?

That all depends on the tooth missing. What you really need to do is get to your dentist and ask these questions.

Its one of the fangs

It could take a while for it to grow in. But without looking at x-rays we can't tell for sure. Ask your mom or dad to take you to the dentist.

I might get a retainer instead so I can remove it when I need to. And I might have to get braces afterword (maybe maybe my teeth aren't that bad I just need to straighten out a few spaces). Would that be a bad idea?

Hon, these are answers only your dentist can give you after he looks at your x-rays. Typically you get braces first and then a retainer.

But without looking at your x-rays we can't give you the answer you are looking for.