The first page in the project page list is always displayed first when you (pre)view the prototype in the viewer.
With the child/slave page currently selected, hold the Ctrl/Cmd key and click the page you want to set as master.
You can change the master page at any time the same way, by Ctrl/Cmd+Click on another page. Only one level of masters is possible.
To remove the master page, simply Ctrl/Cmd+click it master one more time.
There are two ways to preview your wireframe/prototype. The first is a quick one for the current page and opened layers. Simply press the TAB key to activate it, and press TAB again to get back to design mode. This preview is static, meaning that you can't click any of the components on page.
The second preview mode will open your prototype in a new window, and it's actually an interactive preview. All interactions are active as you set them in the design mode, and you can navigate through the prototype. To preview in this mode, press Cmd/Ctrl+J, which will start the prototpe with the default page. If you want to start the prototype from the current page, press Cmd/Ctrl+Shift+J.
All possible actions are also present in the application and there you can see their respective keyboard shortcut.