When you are on a listing page, you can send a direct booking request to the host by clicking on the orange button 'Send request', on the right side of the page.
If you simply want to contact your host and ask them questions about their property, you can scroll down the page and click on 'Contact host' in the About the Host section below their description. Here you will be asked to provide your travel dates, the number of people traveling with you, and a message to the host.
Remember that when contacting the host, they will have 24 hours to pre-approve or refuse your request after receiving your message. If they pre-approve it, you will then have 24 hours to confirm your booking and proceed with the payment.