Designing Combat Areas?

I'm no expert either, but here are some things I tend to do:

• Use small buildings like shacks as hiding spots for pickups and teleporters
• Either put the control point on an elevated area, or in a valley with ledges/heightadvantage all around it. Usually the former for koth/5cp mids and the latter for 5cp lasts.
• Make use of stairs, ramps and raised platforms to keep your battle areas from being flat
• Avoid just throwing props all over the ground. A rock or fence here and there is ok but they shouldn't be the only cover.

Don't be afraid to make mistakes in your first map. No one's first map is good, and it serves as your learning experience. You may get some nasty feedback, but don't take it personally and use it to keep improving.

It sounds like you're looking for a paint by numbers solution, which doesn't exist. Every map is different and there are no concrete rules, just guidelines or common conventions.

Start with an initial idea, then build it in Hammer. Run around the space, and think of how you'd want the paths set up. Put them in and run around again. Think about what would make it cooler. Then put that in, and repeat.

Alternatively, I like looking at cool elements in maps and thinking about how I could use them in different contexts. Like the most famous one (probably), the spire in badlands. How would that work if it was a KOTH point, or a payload cap? Use those directions to guide your design.

The first time you try it, it might not work. It might not even work the second, third, or tenth time. Just keep at it, there are no shortcuts here.

Height variation helps a lot when making an area interesting and fun.