There are two ways you can use WhatsApp on your computer: WhatsApp Web or WhatsApp Desktop. WhatsApp Web is a browser-based application of WhatsApp; while WhatsApp Desktop is an Windows/Mac based application which you can download and install.
To use WhatsApp mobile, you have to download the app on your phone. Go to your iOS store or Google Play market, search for "WhatsApp", install it.
You will need to register your mobile number with WhatsApp before using the service. Make sure you have live internet and 4G/5G connection during registration.