The best investment you probably can you is investing time and effort to know yourself.
So ask yourself the following questions, and try to answer them:
What do you like?
What do you dislike?
What are you good in?
What are you terrible in?
What can you do?
What can’t you do?
What excites you?
What bores you?
What do you need to learn?
What do you need to unlearn?
Who do you like to be with?
Who do you hate to be with?