how do we shard the database for tons of new users, you know, ones that keep coming

Hey everyone, I'm trying to figure out how to scale our main user database. We're growing like crazy and our current setup just isn't keeping up anymore. I'm thinking about sharding it, but I'm not sure about the best way to do it. Sharding by user ID seems easy enough, but what if a few users are super active and overload one shard? Or should I split it up based on something else, like where people are or when they signed up? What's your usual strategy for sharding a database when you expect huge growth?