In double-entry bookkeeping you enter all transactions in the books twice: once as a debit and once as a credit. To keep your debits and credits straight follow this table which shows you how both impact on your various business accounts. Put a copy of this up by your desk to check back to for quick reference:

Account Type Debits Credits
Assets Increase Decrease
Liabilities Decrease Increase
Income Decrease Increase
Expenses Increase Decrease