Is common stock have a normal debit or credit balance?

All Stock is listed under Owners Equity or also known as Stockholders Equity. If you look at the Accounting Equation you understand that

Assets = Liabilities + Owners (Stockholders) Equity

Assets maintain a Debit Balance, while Liabilities maintain a Credit Balance. OE (Stockholders Equity) also will maintain a Credit Balance. Therefore stock will maintain a "Credit" Balance.

The only exception to this rule is "Treasury" stock which is stock purchased back by the company to reduce outstanding stock. Although Treasury Stock is still listed in Equity, it is listed as a negative number (or rather a debit).