Unfortunatelly I have no idea how to force all humans to join one (opposite to bots) team. I can imagine You can do it - like THE_STORM told You - the AMX X plugin which will do this. This is the right place to ask for such question:
http://forums.alliedmods.net/forumdisplay.php?f=12
You need to tell them You are a n00b and You cannot code this Your-self. There are some people they may help You or even they can find for You the existing plugin (if it really exists somewhere).
The only thing You need to know is - how AMX X works and how to add for it a particular custom plugin.