You'll be glad to hear there's plenty to choose from! Most people around mapcore build levels for Source Engine games (Half-Life 2, CS:S, DoD:S, etc...), but there are also people who work with the Doom 3 engine (Doom 3, Quake 4), Cry Engine (Far Cry), BF2 Engine, or Unreal Engine. If you want to go a bit more retro then its perfectly possible for you to make custom levels for Doom, or Quake, or Half-Life, or whatever too.
Are there any games you tend to play more than others? I tend to think its best to get stuck in with something you are familiar with already, as its not really all that important where you make your first steps. If you don't care, I'd probably encourage you to try out the Unreal Engine simply because its next iteration is coming out soon and is going to be huge, but the Source engine is also a good one to learn with due to the vast number of tutorials and active community.
Choose a game and people will be able to give more specific advice ![]()