When you listen to a recording of the piece you want to play, listen to several different ones. You'll notice that it's not the same dynamics for each player.
As a rough guide, pp ( or ppp) is just about as quiet as you can play, to ff (or fff) just about as loud as you can play. Problem is, it depends a lot on the instrument, the room it's in, you as a player, and probably other factors too. And it'll depend on where you yourself put the mid point, from which all the markings are relative.
There's no exact middle point, either. It goes from mp to mf, as there can't be an 'm'. So, it's all rather relative. Obviously f is louder than mf, and pp is quieter than p, but there are no decibel readings for any of the dynamics.
In fact, they're there as a guide rather than etched in stone, and while some pieces sound great when the dynamics are adhered to exactly (although even that is subjective), others can benefit from your own interpretation, or even a re-jigging of it all. Most players will let the music do the speaking, and rise or fall accordingly. We're back to ears again, I'm afraid!