This is pretty old gamemode, which has been in many games before alread. Idea is that, that the players need to capture the enemy's flag and bring it back to their own one.
Added config file in cfg/sourcemod/capturetheflag.cfg
There are right now 3 cvars to change, give me some more suggestions and I will add them.
There are two commands !ctf , will open up flag spawn menu. Don't forget always to prass save (3) and after setting the new map, reload the map, to change the bombsite letter position in radar to your flags. (FLAG: Z) !guns , will open up gun menu.
There are not requiremets really, but there are few plugins that I could suggest you to use.