In general, debits increase the left side, and credits increase the right side (of the equation : assets= liabilities + equity ).
Hence , debits increase asset accounts , credits increase liability accounts, credits increase equity accounts. Income can be thought as increasing equity, so credits increase income accounts. Expenses decrease equity, so it is the opposite of Income, so debits increase expense accounts.
(After a while, it becomes almost automatic to record an expense as a debit to an expense account, and a credit to an asset account (cash) , or a credit to a liability account (payable); a sale as a debit to cash , or a debit to accounts receivable, and a credit to sales account; a inventory purchase as a debit to inventory, and a credit to cash , or a credit to accounts payable ; a loan as a debit to cash in bank, and a credit to bank loan account (liability) .
It is also customary, that debits are written down first , then credits, when recording initially in chronological order in the general journal ).
Once these are gotten used to by practice , there may be also a need to think of contra-accounts, such as contra-asset accounts such as accumulated depreciation, or contra-equity accounts such as Recovered Bad Debts previously written off.