Its pretty easy, well the uploading part. All you have to do, is make a mediafire account (Or where ever, I suggest Media Fire though) go into your saves folder, take the save you want. Make sure you are not in creative or have anything you don't want the players to have, PLUS, make sure you last saved at the point you want them to start at. Next, make a zip folder called Whatever you want it to be, then put the save in the folder, and upload the zip folder.

Now, making a map, I can't really help you with. Making a map is YOUR decision. Because, making maps isn't about the fame or anything, it's about fun, and concideration. It's your imagination. Not Mine. But I would make what would be trending now. Like Parkour Maps are pretty popular. Or Survivial, Like my Zombie Survivial. So, whatever you want. Just make it. Always play the map and make sure it is beatable. If your map requires a texture pack, or a mod. Make sure to tell them to get that mod, or pack!