You don't specify which version of Joomla! you're using but in general you can specify the template in use two ways.
First off you can specify the default template via the "Template Manager" - this is the same in both 1.5.x and the 2.5 series of Joomla!
The second option is that you can specify a particular template to be used for a particular menu item. This can be done in different places based on the version, in both versions you can specify a templates "Menu assignment" by clicking on the templates name in the the "Template Manager". In Joomla! 2.5 you can also specify the template in the "Menu Item" within any given menu.
You can read a some documents on Joomla! templates here.