Plan

You'll spend your whole life being told to plan. And if you're anything like me, you'll ignore it. But with GameMaker this isn't a good idea.

Before I start any game I do two things. Firstly I write out a short version of the story, even if it's just a minigame.
Secondly, I draw. I get some paper and a pencil and I draw the main character in different poses. I draw enemies, levels and more. I'm not a good artist, but as long as it helps me when I come to sprite - it's good planning.

And if you're really lazy and refuse to plan anything, at least do one thing: have a basic plan in your head. Don't just start a game and see what happens - never works.