I was hoping nobody would notice that I didn't have an answer for that - I've never started a conversation, only responded.
/a alliance
/e emote
/g guild
/p party
/r reply (responds back to the person who last messaged you)
/s say (return to normal speech)
/t tell (not sure how to specify who)
/w whisper (not sure how to specify who)
/y yell
Follow the command with a space and then your message - everything except /r changes the default mode of your speech - so once you are talking to your guild you will remain only talking to your guild until you use /s (and a space) to switch back.
(although I found this by trial and error, there are a list of hints if you left-click on the swirly at the bottom of the chat window)
Unfortunately I haven't guessed the syntax to specify a specific person with the tell and whisper.