You can send a message in two ways on HER:
1 - To a Friend
2 - To a Match
You can view your Friends from the Friends tab on your profile, and you can send someone a Friend request by selecting 'Add Friend' on their profile
You can view who has liked you in the Notifications section of the app and you can Like them back by either viewing their profile and selecting the Like button, or hitting the Like button directly from your notifications. This will create a Match and you can message the person from your Inbox.
You can message all your previous matches from your inbox.