Say i want to work on my zombies map with someone over the internet, how would i go about doing that? Do i send them the radiant file? help me

If you both are working on the mapfile itself, then yes. Just send the .mapfile from map_source to your friend and then he can modify it. I suggest leaving compiling to one person though

Use a code-management system like Git or SVN to have a repository of your work.

Popular ones for Git is GitHub (probably my personal favorite now).
While SVN is Assembla but it's all based on $$ now so it's wise to avoid them.

We used Google Drive also, with sync. It was very quick, but did have some hurdles to learn and over come. I used a local svn to backup things checked out to my google sync folder.

We each had a full copy fully working of ORBiT during development up till lately for almost a year. There were many frustrations with that, but it did work once we learned the little idiosyncrasies.


Edit: We also worked in prefabs, so that we could both work on sections and not have to wait for the other to finish a map. The main map was just filled with prefabs for a long time. This also helps with portalling btw. You set up your caulked box or outline of your map, with caulk structure and portal brushes, and then detail brushes inside of it. Very neat and organized. A little frustrating at times when developing layout and entry points.