How do i determine the correct "income Account" from the drop down box , when creating a new customer?

If I understand correctly, you can associate an income account with a customer as the default account for when you bill them. The "correct" account would be determined by you. For example, I work at a healthcare company and we have several accounts based on the type of income, such as Services, IT revenue, Implementation revenue (all accounts we've created to bucket certain revenue together). You may have one or more accounts. We have QB Online and desktop but I only have desktop open at the moment. In desktop, we don't give customers a default income account. Rather, we create sales items, and each item points to a certain income account. So when we create invoices, we select the items we billed, and they point to the appropriate account. You being in the balloon business, you might have items like Yard signs point to income account "signage", item Design Fee point to "design income", etc. Or maybe everything points to one income account called Revenue. The choice is yours! (pop)