1) Before you start travelling you have to think about where you really want to go.
2) Before you start travelling you have to think about what you really want to go.
Which is correc?
1) Before you start travelling, you have to think about where you really want to go.
Only the first is correct. I prefer it with a comma.
Before you start travelling, you have to think about what you really want to do sounds natural, but what you want to go doesn't work.