The accounting equation (assets = liabilities + owner's equity) may help you understand why the revenue accounts are credited...
Assets are on the left side of the accounting equation and the balances in the asset accounts are normally on the left side of the accounts. A balance on the left side of an account is referred to as a debit balance.
Liabilities and owner's equity are on the right side of the accounting equation and the balances in the liability and owner's equity accounts are normally on the right side of the accounts. Balances on the right side of an account are credit balances.
Since revenues cause an increase to the owner's equity credit balance, a credit entry will be required. However, at the time that the revenue is recorded, the amount will be entered as a credit in a revenue account. (At the end of the year the balances in the revenue accounts will be closed/transferred to an owner's capital account.)
To illustrate, let's assume that a company receives $900 at the time that it provides a service for a customer. The increase in the company's assets will be recorded with a $900 debit to Cash (because the asset Cash normally has a debit balance). Since every entry must have debits equal to credits, a $900 credit will also be recorded in the account Service Revenues. (A credit entry in a revenue account will cause the credit balance in an owner's equity capital account to increase.)
If the company earns the $900 of revenue but does not receive the money at the time, the company will need to increase the asset account Accounts Receivable with a $900 debit entry. It must also record a $900 credit in Service Revenues because the revenue was earned. Again, a credit must be recorded in the revenue account in order to cause the credit balance in an owner's equity capital account to increase.
