How do I get a level/map to load after completing another level/map?

Well I don't know how your ending your map, but the basic thing to do is to make a point_servercommand entity. Name it. Then in the Outputs of whatever your using to trigger the entity, have the Target Input on "Command" and have the Parameter be whatever you would type in the console. For instance it could look like, "changelevel mp_coop_lostintransition3" If you are using the exit instances in the game, then your going to have to go into the instances, copy all the info in the instance, find what ends the map, and edit it to look like what I just told you. Make sure you don't actually change instances, but copy it and paste it into your map. Like I made my own prefab to make the exit elevators actually work and make you go to the hub on coop maps by doing this.

Well mine is single player, maybe I should worry about completing the other 21 maps before I start worrying about this. Thanks anyway.

21 maps? If you are making normal exits, then I would deffenatly make that prefab of your exit now so you don't have to go back and edit 21 maps.
Edit
Just thought about it, and your output to trigger the point_servercommand entity would have to be different every time... I guess I would just copy whatever your using to trigger the point_servercommand and paste it into position each map, and then save the rest as a prefab.

http://developer.valvesoftware.com/wiki ... hangelevel

Works great for me so far

Yeah I'll just copy and paste a script and an instance then edit them.