When I was your age I got my first credit card. Then I got a high maintainance girlfriend who turned into my practice wife. Then I got more credit cards. Finally at 26 I parted with the practice wife and went to Bankruptcy Court...

I accidentally got in way over my head over the course of a few years and completely ruined myself financially.
Your FICO score is also called your credit score. FICO is based on what's called the Fair-Isaac score which is a statistical computation of the probability that you will repay the debt in a timely fashion. There are three main credit reporting agencies in the US (Equifax, Experian and Tans-Union), and they each use a variant of this method to evaluate a person requesting credit. Virtually all financial institutions use these credit scores to determine if you're credit worthy, and what level of risk you represent and that determines what interest rate you pay.
Keep this in mind. If you ran up $1000.00 in credit card debt, then cut up the card and never charged another thing, by making the minimum payment, it would take nearly 30 years to repay the debt and cost you many thousands of dollars when it is done!
The posts above are right-on about fraud protection being WAY better with a real credit card vs a debit card. A credit card is also a good way to build your credit if used correctly. Charge something, then pay it off right away. Calyth has a great idea about pre paying a purchase. Send the check to the credit card company at the same time you make the online purchase and you get the benefits of using a credit card without the negative side effects. The other negative about a debit card is that most of them have a daily spending limit which can prevent you from making a large purchase. With Bank of America it's $700.00 per day. This one bit me in the ass several years ago when I was in my early 30s and still recovering from the BK years before. I went to a trade show with my boss and went to check into the hotel but because it was a $150.00 per night hotel, my debit card wouldn't take the charge for the room for a week even though I had plenty of cash in my account to cover the charge. I was quite embarrassed to say the least.
I'm happy to say that today at 40 I've rebuilt my ruined credit and have a top shelf credit score (over 750). It took many years of hard work, but now the only debt I have is my mortgage and an equity line of credit that I use for things like auto purchases so I don't have to deal with the car company's high interest rates. I have a couple Platinum cards with high limits and NO BALANCES! I use them regurlarly for purchases both large and small, but pay them off in full each month. This helps me to get more rewards points!
The rule of thumb I use that I learned after Bankruptcy is that if I don't have the money, I don't buy it. With zero credit after BK (no one would extend any to me), I had no other option.
One more thing, these days many of the better jobs require a good credit score. The thinking is, if a person can't be financially responsible in their personal life, what kind of employee would they make? A guy with a poor credit score is more likely to steal, have car trouble he can't pay for, etc...
Credit is an important part of modern life, but is a double edged sword. Like anything sharp and pointy, be careful.
jmx
PS: Where's Gollnick when you need him? He'd have saved me a lot of typing here by explaining this all before I even saw the thread.