To add to @MahGah's response....
Before you load customers and vendors, you should create your customer posting groups and vendor posting groups. If you plan to have inventory then also you need general business posting groups on each customer and vendor. You need payment terms, currencies (if you need them), salespeople/purchasers (you might want), and don't forget contacts. If you are using dimensions and want some values to default from customers and vendors into your transactions, that's another consideration.
As you look at the configuration package, many of the fields have table relations so consider if you need or want data in those tables as well.
