To bring your site online, open the Sales Channels section in the top bar of your control panel or from the Dashboard. To publish your site, you have to click the grey button next to the sales channels to enable it, making the button go from grey to green. This means that the sale channel is online and visible to your clients.
When the site is offline or in maintenance, next to the sale channel you will find a maintenance password to insert in order to see the site preview. The maintenance password is provided automatically from the system, but you can change it.