How to change the default Joomla template and assign different templates to menus and menu items
Templates can be switched from the Template Manager of your Joomla admin panel. First log in to your admin panel, then either click on the Template Manager button or go to the Extensions menu in the upper left corner and select Template Manager. In the Template Manager you'll see a table with all the templates, both for the frontend and the backend. The default ones are marked with a star in the Default column.
To change the default template either mark the checkbox in front of the template you've chosen and then click on the Make Default button in the upper right corner, or click on the empty star icon in the Default column.
To assign templates (different than the default one) to menu items, in the Template Manager either click on the template's name or mark the checkbox in front of it and then click on the Edit button in the upper right corner. Then, on the edit page you'll see a panel labeled Menu assignment. Mark the checkboxes in front of the items that you want to set to use the particular template. After that click on Save & Close in the upper right corner.
You can also check out the tutorial on changing the default Joomla template and assigning templates to menu items.