Details depend on the OS of the smartphone. Those are different for Android (maybe even for different versions of Android), iOS (on an iPhone), Windows Phone and possibly Blackberry. So if you tell the OS of your phone you'll get better help.
In general, the process is as follows:
1. Go the app store/play store (whatever it's called)
2. Search the app you want
3. Download it
But why not ask a friend to show you exactly how it works? Since 'everybody' has a smartphone nowadays, it shouldn't be too difficult to find somebody that can help you. No better help than help in person.
Kees
Presently, Every Person use Mobile Phone, but How to Download Apps and use in Smart Phone please describe, because i have never idea how to use it.

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