Apart from drupal's core modules which is already quite complete to create a powerful website, there are other contributed modules available that is maintained by the drupal's active community to enhance drupal's usability.
There are lots of contributed modules available at official drupal contributed modules, you can select the desired module to install in your drupal website to extend the functionality of your website. Drupal's contributed modules are easy to install, but before installing any new modules to your live drupal website, be sure to go through the module's release notes to ensure there is no compatibility issues, and if there is any special steps involve to get the modules installed. A good and recommend approach is to install it to your test site to test it thoroughly before installing it to your live web site.
Installing drupal modules involve three steps :-
* downland and extract the desired module compress file
* extract and place the extracted folder in the sites/all/modules (***See note below) folder in your drupal website
* Login to your drupal administrator screen administer > site building > modules and activate the newly installed modules
NOTE: sites/all/modules folder is for storing contributed modules, /modules/ folder is reserved for drupal's core module, this is to separate the drupal core and contributed modules to avoid trouble in future upgrade maintenance.
Extra note: you may want to also check and assign appropriate user access permissions administer > users > access control for this module in your drupal website.