The first chapter of the World Builder's Guide offers some useful suggestions regarding how best to get started. It really depends on how you think about your world, so there are various ways to approach it. That chapter outlines five different ways, and it's highly likely that one of them will feel right to you.
Chapter 2 of the Tips & Tricks Guide also has some helpful suggestions about getting started. It's a very short chapter of a guide that is packed with incredibly useful bits of guidance to get the most out of the product.
After you've reviewed those two chapters, you may have further questions, but they'll be much more specific than just a general "how do I get started". So pick the approach that feels right to you, start experimenting, and post whatever questions arise here. I'm sure you'll get some very helpful ideas on where to go next.
But the first step is to pick a general approach to get started.
Hope this helps!