how do you decide when to cache, and where

We've noticed our performance takes a hit when things get busy, and caching looks like the logical next move. But man, there are a lot of choices out there. You've got in-memory stuff like Redis or Memcached, CDN caching, caching database queries, and even browser caching on the client side. So, how do you guys figure out which caching strategy to use for a specific piece of data or an API endpoint? What's your usual rule of thumb for picking the right type and level of cache?