To set payment terms for a particular customer so that they are applied automatically when you create a new invoice for that customer:
Go to the 'Contacts' section
Open an existing customer or create a new customer
Select the default payment terms for the customer
You'll also find the option to set payment terms while creating an invoice or quote - note that even for customers with set payment terms, these can be adjusted on a case-by-case basis simply by changing the payment terms directly in the invoice.