Prospective investors carry too much credit card debt. This mistake is a corollary of Mistake #1.
Stay out of credit card (CC) debt. Mistake #2 is not so much an investor mistake, but rather a mistake of bad management as it has nothing to do with managing a portfolio. If one ever aspires to be an investor, you must get the CC monkey off your back. Pay down this debt to zero before you begin your investment life as the interest charged on the credit card is likely more than one will earn in the stock market.
The earlier one can get out of debit and begin building a retirement portfolio the better.