Which one is correct I think you would be a good teacher or I think you will be a good teacher?

Welcome to Fluther! Both statements are grammatically correct, although they imply different meanings.

“I think you would be a good teacher.” This statement implies that career advice is being given. “I think you will be a good teacher.” This statement implies that it is providing encouragement to someone who decided to pursue it, but has not yet started the training or a job in that field.

You are using conditionals.

This will explain the proper usage:

http://grammar.ccc.commnet.edu/grammar/conditional2.htm

The first sentence is in in the conditional tense, meaning a(n) hypothetical situation that has not occurred yet and might not occur; you are just speculating about it.
The second sentence is in the future tense, which implies that the situation will eventually occur.
Both are grammatically correct.

How about “I think you could be a good teacher”? Suggest ability rather than will.