In single player you cannot open the chat box without mods. In multiplayer you press 'T'.
Chat with our AI personalities
It is not possible unless you download the single player command mod.
The deafult key to open the chat is T.
Yes with LAN. Just get your friend to open a single player world to LAN and then click on multiplayer and select his world
If you have cheats turned on (which you do when you create your world), open the chat box with t and type /weather clear
It is multiplayer, but also has open single player available for you to play around with.