There are four main means of communication. To send a message to everyone (usually the best way to get a question answered!) just use the command SHOUT followed by your message in quotation marks, eg. SHOUT "I'M CONFUSED!". Most commands in MUD can be abbreviated, and this one can be shortened in two ways: firstly, SHOUT can be reduced to SH; secondly, the terminating quotation mark can be omitted. Hence, SH "I'M CONFUSED is the best way to do it. Shouted information is usually received as coming from 'a male/female voice', so people can't always tell who shouted. You can never be sure of the reaction of other players to a shout, so if you are ignored or they shout back sarcastic messages, don't feel bad about it!
The second means of communication is more local: you can talk to everyone in your room by using a string without any command, eg. "HI FOLKS!". Again, you need not type the final quote. The message goes to all players in the same room as you, and they can tell who said it because it appears as 'X says "HI FOLKS"'.
The third way to communicate is on a person-to-person level: to talk to an individual player, give their name (NOT including their rank) followed by the message in quotes, eg. PERCY "WHERE ARE YOU? (the final quote has been omitted, as usual). Percy would know you had communcated directly to him because he'd see something of the form 'X tells you "WHERE ARE YOU?"'.
The final method of communication is via WISH, as in WISH "PLEASE HELP, I'M NEW". This sends your message only to the most powerful (and friendly!) players, known as "wizzes" (wizards/witches). If there are any playing, even invisibly, they'll receive your message and will normally come to your assistance; a message from 'someone powerful' means it is from an invisible wiz. WISH is actually a spell, and you usually fall asleep when you use it, so don't be surprised if this happens.