How can I create a box where people enter a donation amount

You have to add Paypal (https://www.jotform.com/help/50-PayPal-Forms) in your form and make sure to select Collect Donations/Custom Amount Payments in the configuration:

If you want to use a textbox field, you can also do that and then pass the amount to the payment field. Here's our guide:

https://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field