Visit the Parallels Online Store to buy a Parallels Desktop subscripiton:
Click on Upgrade Now or Buy Now, depending on whether you have a license for one of two previous versions or not. Visit KB 115399 to find out what type of license you need.
You will see a new window with the available licenses. Click on the purchase link next to the plan you choose.
Fill in your contact and payment information and complete the process. You will get your activation key shortly after the purchase is completed.