You can find some info on Map theory here
http://tf2wiki.net/wiki/Maps
And also, I recommend using A Boojum Snarks' Gametype pack, it's basically a .vmf containing all the entity work behind all the TF2 Game modes, just copy the game type into your map and Voila!
Here it is: [ame="http://www.tf2maps.net/forums/showthread.php?t=4674"]Ultimate Mapping Resource Pack - TF2Maps.net[/ame]
Although, it is also good to learn how it all works, so after copying it over, you may want to have a look at what it actually is, and get a grasp on how logic entities and the like work.