To be able to send someone a message and chat, you first need to match with them.
A match is a mutual like. When you like someone, and this person likes you back, there's a match!
Tip #1: You can use Rainbow Likes to make your like stand out and increase your chances of a match.
Tip #2: Send a message/icebreaker along with your like to call more attention. Keep in mind that you'll only be able to chat after the other person likes you back.