well in the past level design more or less was creating the world in which stories are told. In the future level design will be more gameplay oriented with different people dealing with gameplay, scripted sequences, and other art assetts. So if you want to make levels I'd suggest downloading the Source SDK through steam, going to http://www.snarkpit.net and read some of their fine tutorials. See what you can come up with. Make a couple of test "maps" learn the tools, and than create something unique and exciting to show off. Thats the best way to learn. Have peopel critique your work and be harsh to you. Thats how I'm learning to model right now. So I guess thats my number one tip (yes i'm not the best at it sometimes...wacko) But look for as much critiquing as you can, and listen to everyone's opinion. Cause it'll help you soooo much in the long run.
Goodluck!