The best way to learn is to play people's campaigns and see how they did their triggering. Especially new campaigns that are one or two mission into it. You can watch as they develop and pick up tricks and ideas. Don't get ambitious on your own maps at first...especially with terrain or data. Just try to make something similar to one of your favorite Blizz maps or custom campaign maps. Heck, you can just take any of the 100 of Blizz maps and use their terrain and make you own mission on it!!
Make sure to leave feedback on custom campaigns so once you make your own maps people will want to help you out by playtesting and giving good critiques.