Besides what FMPONE said, play lots of game from different genres and try to understand the gameplay and how the levels are made. Research is the key. Also it would be better to choose either Level Designer or Environment Artist when you start. Doing both at the start will only increase your confusion. I recommend you take a look at www.worldofleveldesign.com.
Also take a look at beginning level design book. Its is good for beginners.
You can also start meddling with UDK just to get a hang of it.
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