You should enter all the correct balance sheet account (asset, liability and equity) balances at 12/31/17. This includes the bank balance. If you are a corp or partnership you can get the balance sheet balances from the tax return. If you are a sole proprietor, it's not on the tax return, so you'll have to reconstruct the balances. The bank balance is easy because you can use a bank statement, adjusted for open items (eg, a check mailed on 12/31 that cleared in Jan). You can enter all the balance sheet balances using a Journal Entry. Assets are debits. Liabilities and Equity (including Retained Earnings and last year's profit) are credits. Of course it should balance. That's why it's called a balance sheet.
To just enter the 12/31 bank balance, you can offset it to Opening Balance Equity, pending later research and correction. OBE should be eventually be zero. If there is already a 12/31 bank balance in the books, then enter the difference to force the balance to agree to the statement, adjusted for open items
