how to installing application

The App Store should be installed as part of the OS. Have you checked the Applications folder?

If necessary, do a backup, preferable 2 separate ones on 2 drives. Boot to the Recovery Volume (command - R on a restart). Run Disk Utility and select First Aid. Then re-install the OS.

Reinstall from macOS Recovery.    

OS X Recovery.    

ASC YouTube for how to reinstall macOS from macOS Recovery