Hi @Ianhere,
There's not too much more to it than what the others have said. You look up the app on your relevant app store, like Google Play or Apple store, then install it, and follow the set up instructions when you first open the app.
If you're running into some sort of error message please tell us more, or check the options on the app store page to get support from Whatsapp. I'm not aware of any common issues with setting Whatsapp up though, you just need a mobile data or Wi-Fi connection, and to be able to receive a verification text to your number.
Hope you managed to get this sorted in any case.
Thanks,
Jonathan
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