There are a few different ways that you can open your chat in Minecraft. The most common way is to press the 'T' key on your keyboard. This will bring up the chat window and allow you to type in whatever message you want to send. You can also use the '/' key to open up the chat window, but this only works if you have cheats enabled. If you're playing on a server, then you can also use the '/msg' command to send a private message to another player.