Practice. Look at other people's levels. Practice. Oh, and did I mention practice?
Look at me, my first few levels were so bad a baby wouldn't even have wanted to use them to gnaw on'm.

And now I'm in a team of level-creator experts making levels of magnificent quality (if I may say so, nyarhar. X3 ).
That took me about 5 years. (With about two years of nothing at all. )
First, understand the basics.
Then, experiment.
ANd THEN you MAY want to try to do something BIG.
Take your time if you want to become good.

So much from me. Narf.