Pretty much anything that makes your muscles feel sore and doesn't cause injury is worth doing.
IMO: most of what makes fit people fit is becoming aware of their body in detail: how to use muscles, how they work, what they feel like, what they should feel like when they're working together -- all of this builds a foundation where, if they lost their fitness, they would just get it back again.
So if you don't know what you're doing: just learn by doing. Pick something that it seems like your body should be able to do easily, but can't, and fix it. Repeat x100 and you'll be fit as hell.
You can start with bodyweight though. Pushups, pull-ups, etc. I recommend the YouTube channel FitnessFAQs or Calisthenicmovement for a good start there. No nonsense, direct guides with discussion of technique and injury avoidance.
IMO: most of what makes fit people fit is becoming aware of their body in detail: how to use muscles, how they work, what they feel like, what they should feel like when they're working together -- all of this builds a foundation where, if they lost their fitness, they would just get it back again.
So if you don't know what you're doing: just learn by doing. Pick something that it seems like your body should be able to do easily, but can't, and fix it. Repeat x100 and you'll be fit as hell.