Best piece of advice you can give someone starting out in Level Design 
Go!pic.twitter.com/UU5IC5mDDh
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Observe everywhere you go. Really observe it. Layouts, decorations, even the paths people take (including the ones they shouldn't). Try and recreate somewhere you're familiar with and add your own twist.
Oh and take photos of anything you see that's useful for reference. I used to do this all the time and it was a great help.
