To install an app (a small program for your phone), open your phone’s app store and search for its name. It is free, unless the listing shows a price.
Open the App Store (blue icon with an A) on iPhone, or Google Play (coloured triangle) on Android.
Tap the magnifying glass, then type the name of the app.
Tap Get (iPhone) or Install (Android). You should see a small circle filling up.
When the circle is done, tap Open. You should see the new icon on your home screen.
If you don’t see it: check that the phone is connected to the internet. If the phone asks for an Apple or Google password you have forgotten, tap « Forgot password? » on the sign-in screen, then follow the message sent to you by email or text.
Limitation: several apps have almost identical names. Compare the publisher’s name, written under the title, with the one you were given.
To picture it: an app store works like a pharmacy counter: the products come from a controlled source, rather than from an unknown website.