You must be using a google chrome web browser and make sure that you are running the latest version of WhatsApp. If yes, then follow the following steps to use your WhatsApp on PC/computer:
1. on your computer, go to chrome browser and head to HTTP://www.WhatsApp.com and download WhatsApp setup from here.
2. Wait for the download to finish and then Install it by going to download location and clicking on WhatsApp setup.
3. Go to WhatsApp on your phone, go to settings and select WhatsApp web.
4. Using the phone's camera, scan the code which will verify your account. The installation process is completed.