complete installation proess for drupal 7.15 and ubercart-7.x-3.0
Note: This is an alternative way by using Drupal commerce module and not the Ubercart module.

As a beginner I'd suggest you should try the Commerce Kickstart distribution first. It includes the latest versions of Drupal core, Drupal Commerce, and the other modules it depends on.  You can explore Drupal after that ;)

Just make sure you select the Commerce profile while installing (see screenshot)

And also create the example content after configuring the site