Can someone guide me how to install this app and build app
Hello @alexchrishanthan and welcome to the DHIS2 community. 
If by app you mean the DHIS2 system, you can learn how to host and configure it in the docs. You can start by reading here.
If you want to test it out without installing it (recommended), there are demo servers for each supported version found here. Default credentials are provided in the login screen.
Regards.