What most people do is they make a map with a gamemode that they like. For example, I like payload the most, so generally my maps are payload. I may attempt an attack/defend cp, another koth or a symmetrical cp one day though.
Once you've chosen a gamemode, think of things that make a map that you like, or an interesting idea of a layout or detail that you'd like to implement, or something that you'd like to improve on. An example would be a 5cp map that encourages pushing rather than holding a point.
Also just try to make a map that you think would be enjoyable, then put it up for testing, get feedback on it, change things to accommodate, and test again.
There's a whole bunch of guides online for stuff, like
crash's video tutorials, but a lot of the stuff that you need to learn can really only be learnt through making maps.