When a support request arrives from a customer, it will appear in your inbox in the "Open & Unassigned" section of your inbox. Click that link to see all of your Open tickets.
Click on a ticket and open it and reply like a normal Gmail. Your response will be sent to your customer and synced across all your other agents' accounts.
By default, we also automatically close the ticket when you reply. You can adjust this in your settings, or choose to leave it open when you reply by clicking the arrow next to "Open".