PvP help

Ok, so far Guild Wars is one of the best games i have ever played. However, i only had a chance to jsut try a little bit of PvP at the end of the last BWE. I created a guild and had a few members in it (mostly my friends) but i never really knew what to do with it. How do you actually have a guild vs guild battle? Because all of the things i went into it was random teams. Really fun though. I am just asking for some more understanding of guilds, guild vs guild, and PvP. Thanks alot.