What is the entry to post when setting up a new bank account for the beginning balance?

There is no journal entry required when establishing your account, only when you deposit or withdraw funds.

The entry to record a deposit is:
Debit Cash for the amount
Credit Accounts Receivable for the amount if a customer is making a payment, or Sales revenues if Cash received on a sale, or appropriate Account.

The entry to record a withdrawal is:
Debit the appropriate Expense or Payable account for the amount
Credit Cash for the amount.