Ubuntu comes with Ubuntu Software Center, a system which inspired Apple's Mac Store. You can find Ubuntu Software Center in the Launcher. Open the USC and go to 'Edit' > 'Software Sources'.
* Here under the 'Ubuntu Software' tab you can change the server location to your country or to the main server. All options, other than 'Source Code' are enabled by default.
* Move to the next tab 'Other Software', here you can enable the option 'Canonical Partners', leave other options as they are.
* In the Updates tab, select the Important and Recommended updates. Don't enable the pre-released updates. By default 'Unsupported updates' is enabled, you can keep it enabled. Keep the settings as you see in the following image.
Authentication & Statistics are the next windows which we will not be needing for now.
Now click on the close button and it will automatically update the changes. Now, your repositories are set and you can start installing software.