The main thing you want to do, is to try and avoid making it one straight box. What I do when trying a layout is place a few square brushes and try to think, "What does an attacker get of this route, what does a defender get, etc." After a very basic layout, I start to raise/lower the brushes, and fine tune a bit more what the layout is. Adding height is very important for a map, and without it the gameplay is stale. After height and layout are feeling okay, start adding displacements! Don't make my mistake of a flat room, with everything being smooth
View: https://www.youtube.com/watch?v=nre432l_OTc
This video covers the fundamentals of them. One tip is to try and never have only one way to an area. In this map for example, you can only walk forwards to get to the point, there's no other way (leading to bad gameplay). Viaduct is a great map to get an idea of basic KoTH gameplay, and I suggest using the Viaduct formula for your first maps. Once you have experience, then try to break that formula, and make new and exciting stuff. Finally, you want a good scale. Place down a prop_static, and change to a player model (Most people use player\heavy.mdl as that model) and use it to see just how big everything is. I'm not an expert on mapping, but hopefully this helps a tiny bit (Also watch UEAKCrash's mapping guide, 10/10 series)
