Is it worth to join any institute

A 750 score is the 98th percentile - that is, only 2% of the entire test population scores at or above that level. It is incredibly difficult to achieve such a score through self study. (It's incredibly difficult to achieve that score at all, let alone through self-study.)

As with any topic, there are huge advantages to studying with an expert vs. studying on your own. That doesn't mean you can't or shouldn't study on your own. But any given individual is likely to do better if s/he studies with someone who really knows how to teach the material.

Also, be aware, no school requires a 750 (they wouldn't have very many people to admit if they did). It's nice to have a high goal... but there's really no reason why anyone has to score 750+. (Unless you want to teach for us someday...)