First you want to download the skeleton gamemode, which is basically just a blank gamemode, like if you made a new lua script it would be a blank script. For myself, I have a custom Sandbox gamemode which is REALLY similar, since you sound new, you shouldnt make a new one from scratch yet, you might want to make a copy of the sandbox gamemode folder, and edit and mess around with it, thats where you can start. If you want to start making one from scratch then,
download skeleton gamemode here:
http://www.garrysmod.org/downloads/?a=view&id=39584get codes for scripts here:
http://wiki.garrysmod.com/page/Category:Gamemode_Tutorialsgood luck with the gamemode, you should probably want to hire someone from coderhire to make your gamemode
Edit: I posted this right when cobalt did, so yeah. lol