I have already signed up but I don't know how to purchase

Click the Pricing link in the navigation menu or you can hover over your account menu, and click the Upgrade Your Plan option as shown below:

Select the plan you wish to buy and the billing cycle (monthly or yearly), then proceed to pay with a valid credit card or your PayPal account

And that would be it. Here is a guide with a more detailed instruction https://www.jotform.com/help/331-how-to-upgrade-my-account