Build your emergency fund
Regardless of age, building an emergency fund covering three to six months of living expenses is the essential first step. At 21, your expenses are typically low, which means the three-month minimum is probably sufficient for now, especially if you don't yet have dependents or a mortgage.
Start small. Even $10 or $20 a week directed into a dedicated savings account adds up. As your income grows, work toward the six-month target. Keep this money in cash, not the stock market. Emergency funds need to be immediately accessible when you need them. Motley Fool Money keeps a current list of the best high-yield savings accounts, so you can make sure your emergency fund is earning a competitive rate while it sits.
Avoid credit card debt
Student loans are a major source of stress for many young people, but credit card debt is typically far more damaging. The average interest rate on a carried credit card balance is above 20%, significantly higher than most student loan rates.
At 21, you haven't had much time to accumulate a large credit card balance, which works in your favor. Pay off whatever balance you have once your three-month emergency fund is in place. Using a credit card isn't the problem, since it's one of the best ways to build a credit history. The problem is carrying a balance. Only charge what you can pay off in full at the end of each month.