I didn't want to make a whole new civ so all I did was copy the american civ and paste it at the end of the xml file, I only changed the name form CIVILIZATION_AMERICA to CIVILIZATION_CUSTOM but when I start a new game its not there. I didn't make my own folder, I just fodified the default warlords xml. What else do I need to do?

It sounds like you changed the type and not the name. The type is the internal name, used by the game (you really should change this one, like you did). The description, shortdescription, adjective and civilopedia are the things you see in the game. So if all you changed is the type you should now have two america to choose between.

I just fodified the default warlords xml.
I would really advise against this in future. Use the mods folder or CustomAssets. For something samll like this, it should be all right if you do a backup, but for anything major I wouldn't recommend modifying the original file.