For a first map with gameplay I recommend either Arena or CTF if you want to make a symmetrical map, or Attack/Defense if you want to make a asymmetrical map.
Arena maps are the easiest maps to make since they are required to be smaller by design, which means less map you have to make, they are also pretty easy to balance as all you really need to do is create a fun combat arena without worrying too much about flow.
CTF is good too if you want to try something different, CTF layouts can help you understand player flow and navigation more, plus it's not as oversaturated as KOTH.
Attack/Defend is by far the best way to start out as you will learn more about pacing and flow as well as not having to mirror every single change to the other side of the map every time you want to make a change.
KOTH is deceptively hard to make play well, and even harder to come up with something unique that as not already been done before 10 times over. I think most people are tired of playing koth_firstmapever every other day