Think writing Fantasy will be easy because you can just make everything up? Think again! Yes, you are not bound to our laws of science. BUT your world needs to have its own logical consistency. (In both Sci-Fi and Fantasy, this is called world building. Check out our page for Sci-Fi also.)
Fantasy writer Patricia C. Wrede suggests you start by answering some of (not necessarily ALL of) these questions to help with world building for fantasy. Here’s another list developed for Dungeons & Dragon’s Game-Masters that might help you.
Once you’ve figured some things out, you might want to decide what kind of Fantasy you are writing. Here’s a list with links to wiki articles on each type.
You’ll probably want a map of your world. Holly Lisle has these suggestions for creating your map. Here’s some quick and practical map making advice. More dedicated cartographers should check out this tutorial from Jessica Khoury, author of Origin.
Want help creating the backstory? Check out this game you can play! (You’ll need a deck of cards and some D&D dice.)