After starting and re-starting my first project a few times, I found starting with tectonic plates to be the easiest way to go. Draw about 3 largeish and a few medium plates with shared borders (about 5-7 total including 1 arctic plate), and then figure out the directions they're going. Different meetings of plates cause different features, there's good resources here and around the Internet on that, and that'll give you a basic idea for mountains, chasms, deepwater trenches and a few new island chains. Then just draw some vague shapes based around where you know you want certain features, and piece the rest together like a puzzle taken apart, based on tectonic movement. Voila, instant world. Then just remember basic stuff like deserts at 30 degrees, rain shadows, and rivers start at mountains and flow toward the ocean, and you should have a fairly believable world.