Go to your options section at the bottom of the green bar that runs along side the left of your program. Then choose the payment terms tab. Double click on a payment term or enter a new one and then check the box "default payment term for new customers"
Then you can go to your customer section choose a customer and click the edit button. Click on the billing tab and set the customers payment terms.
If you want to change the payment terms of an invoice that is already created, go to the invoice section, double click on the invoice and then at the bottom of that window, change the payment term drop down box.