Minecraft wiki entry for Playing_on_servers, associated text: You will spawn in the server's spawn point. By default, servers have a protected area 16 around the spawn where you won't be able to break blocks or interact with stuff.
On the left hand side of the screen is the chat, where people can send messages. The default key to type messages in the chat is the T key. You can also type commands into the chat, which does various things. For a list of commands available for you to use, type /help