I would be very thankful to you if you could tell me the meaning of two phrases.

First "it help me get straight teeth". I think this could mean a big smile.

and secondly "would you fix your teeth"

Grateful for your thoughts. Thanks.

It helps/ed me get straight teeth.
I assume that this sentence is referring to braces, the orthodontic device usually worn by young teenagers (although worn by adults more commonly these days) to help teeth grow or point correctly. We say that teeth grow straight when they grow correctly out of the gums.

Would you fix your teeth?
Here's another example of how difficult it is to offer an accurate explanation without context.
Fix your teeth means "have some kind of dental work done on your teeth." It could be anything involved with dentistry, orthodontics, or periodontics.

quote:

"would you fix your teeth"

Richard's explanation is right, of course.

But, I don't think the original sentence is a natural sentence. It is more likely that we would say:

Would you have your teeth fixed?

We'd use this passive causative over the active causative since presumably the speaker himself / herself is not going to do the fixing. Someone else -- a dentist -- will do the fixing.