Wow when I was 12, I was just starting to play d&d and we didnt even bother with maps. We did everything off the tops of our heads, and most of the time we didnt even use dice.
So basically like everyone has said, keep it simple. If you feel you need a map use graph paper, keep the scale easy, and the details simple.
A good rule of thumb is to simply think about the key areas your characters might encounter battles in. Have a definite idea of how big that area is, and what is in it, that way if you need to make a quick battle map sketch, you will know what you are working with.
Don't let rules bog you down, if something is giving you problems, think of a quick solution you can handle, or better yet, if you know ahead of time that something is going to give you problems, just get rid of it for the time being.
On mixing plots, and creating ideas. A flowchart or bubble table is always a good place to start. Create the idea for the mission. Say, someone has been kidnapped. Create the tie in that draws the players to the mission, perhaps a bounty is offered and they hear of it. Who is the main villian, make sure you have stats for the villian and any henchmen they have. Create the ways the pc can solve the kidnapping. What npcs have what information, what clues exist, things like this. Work it through in your mind and make sure it is easy enough that the players can follow through on it without stumbling around.
I think you will do fine. It sounds like you want the players to have fun, and just keep that in mind and I am sure you will do great.