Easiest thing is having both two of you connecting to the same already-existing server created by someone else. "Callvote", if enabled, may allow you to change gametype/map...
Of course you can create your own server, but
bad news it that you have to configure your router and firewall(s) to allow incoming connections on a certain UDP port first (usually UDP port 27960). After that, the rest is relatively simple: check your own current IP address, (
http://www.whatismyip.com/ or similar site can help you), then launch the game and create your own server (multiplayer->create), then tell your IP to your friend and he will use "/connect ip:port", like "/connect 10.0.0.1" or "/jconnect 10.0.0.1:27960".
Fore more info, you can refer to:
(
DO NOT LINK) h t t p s : / / openarena . wikia . com/wiki/Manual/Multiplayer
(
DO NOT LINK) h t t p s : / / openarena . wikia . com/wiki/Servers
(
DO NOT LINK) h t t p s : / / openarena . wikia . com/wiki/FAQ#How_can_I_organize_matches_with_my_friends_all_around_the_world.3F_How_can_I_talk_with_them.3F