Look for tutorials, they help a lot. And other maps it they are not protected.
First, learn how to make a good terrain, with various tilesets, matching doodads.
Second, make new units, heroes, with unique abilities. Also search models and icons for them.
Third, plan the gameplay. This is going to be the hardest, learn how to make triggers, and Jass. Tutorials, and the world editor zone forum can help you a lot.
Try to make the map replayable. Make it entertaining, you can make cinematics in the beginning. make some different heroes. If you make a campaign, make entertaining and interesting quest, not repeatitive.
I hope this helps. If you have further questions, feel free to ask.