Joomla comes with several built in modules which you can add to your site. To do this simply go to 'Module Manager' in the extensions menu, click the new button then click on the module you would like to add. Any 3rd party modules which you have added will show up here also.

You will then need to configure the module which will vary depending on which module you chose however every module will need to be assigned to a position. If you have multiple modules in one position you will then need to choose in what order you would like the modules to be displayed. Both tasks can be performed in the modules 'Details' section. For a diagram of the module positions available see the diagram on this page http://www.onlineworkfromhomejobs.net/modules-introduction

In the 'Menu Assignment' section you can then choose which pages you would like the module to show up on.

Another option which you will find in all modules is the field 'Module Class Suffix'. This allows you to choose how you would like the module to appear. Most templates will come with several different preset styles for your modules. Check out this page to view the different options. http://demo.rockettheme.com/apr08/

You now know how to add a module.