You can find your chats at the top right corner of the main screen, by tapping the icon with two speech bubbles. There, you’ll see all your existing conversations and will be able to start new ones.
If you want to chat with a specific user, you can also search for them via Menu > Find players. After entering their name, you’ll be able to access their profile and start a chat.
You can also start a chat with someone if you’re currently in a match with them. In that case, you’ll see a chat icon inside the game.