just build the map - , and add the DOM, CTF or AS prefix to the title.
If you want a domination map, you need to add control points (in classes->navigationpoint. You can name your control points by right-clicking on them, and adding a name in the position section of the controlpoint tab in the properties.
For CTF, add a flagbase, also in classes->navigationpoint.
I wouldn't worry about assault until you are a kickass mapper, because they are really
hard to do. If you do do assault, you need to use fortstandards(under classes->pawns-> stationarypawn).