I was thinking about creating my own adventure map, but I don't know how to start! Do I press certain settings? how can I get my map the only thing in the world? (like if you fall, you fall of the end of the world) And when I'm done, how do I make it downloadable? (Please include all i need to know about making maps, like how to make you spawn in a certain spot in the map, etc.) Thanks so much!
Alright! You need a working knowledge of WorldEdit or McEdit. To make your map the only thing in the world, you would need to use a 3rd party program known as McEdit. In fact 95% of maps use McEdit. If I were you, I would wait until you got familiar with the technical side of Minecraft. If you are asking these questions, you are not ready.
Do you know how to install mods? If you cannot do that then you will not be able to create a custom map.
Another option is using the FlatLand option when you generate your world. There are no extra settings. Once you are done with the map, go to your saves, zip it and upload to a site like MediaFire.