Good advice there, George MC.
Start simple, start small.
Look at other people's work and learn. Read the excellent guide supplied by BFC, but don't feel pressured to do everything in there perfectly.
Pinch other peoples maps and adapt it for your purposes, or crop some piece of a supplied Master Map. Huge time saver.
Don't be tempted to start with the Mother Of All Battles. You will likely never finish it. Waste of energy.
Making scenarios is fun, but it takes some practice.
But it can be done (heck, even I took the plunge with my limited skills, so everybody can).
And once you finish your puppy, you feel on top of the world