This is the beginning of your relationship with a new customer so keep the tone of the letter friendly. Let the customer know when he will (or won't) be approved for credit.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
We are very pleased that you have decided to do business with Abc Company and look forward to serving you in the future. Before we can establish a credit account, we require all new customers to complete the enclosed credit application. Once we receive the completed form, we can set up your credit account.
You should hear about your credit approval <time period>. If you have any questions about the application or our approval process, please call <name of contact> at <phone number>. Thank you for deciding to do business with Abc Company.
encl: <List of enclosed items goes here>