Send him(her) a booking request by indicating the dates of your trip and the number of travelers.
Don't forget to give some details about your future trip, why are you coming, your hobbies, etc.
In order to increase your chances to be accepted, send booking requests to 4 different hosts at least.
But be careful, if you register your credit card while your booking requests, you'll accept the automatic debit and so you'll be charged for the first host who accepts you.
So, don't forget to cancel your booking requests if you find another place to stay (hotel, hostel, etc.).