I believe the default is the "t" button. Just in case you didn't know, like danniel said, use "/ch g" to speak in global chat. You can also use "/ch l" which will only allow players to hear you in I believe a 100 block radius. Other chats include town chat, /tc (message) and nation chat, /nc (message). I do believe it you type "/ch help" it will give you a list of chat commands.