Sundownerz, I started my travel as a college freshman. I echo toomanybook's words about capitalizing on the time you have! I will venture a guess that you're hoping to see as many places and live as many experiences as possible.... that's the perfect formula for mileage running in its purest form: planning travel based on where deals are found.
Programs: Learn the alliances, and unlike the advice of an earlier poster to sign up for every program, I suggest you pick a couple and focus on them. Alaska Airlines Mileage Plan allows you to pool miles flown on American, Delta, and Alaska. A United or US Airways account can be used for travel on both. United lets you redeem for one way flights, so all else equal, a United account will reap rewards sooner than US Airways. Those 2 programs, United and Alaska, will cover the lion's share of flights in the US. If you fly Southwest, then jump into Rapid Rewards, but understand that because they credit based on dollars spent, you will have to fly many many cheap tickets before you see rewards. By and large, Southwest will probably be one of the less generous reward options for you.
Credit Cards: As for credit cards, I would suggest you ignore all the credit card talk-- while you might end up with 1 or 2 as backups, follow your parents' guidance for building your credit history, and be very careful with your choices. As a traveler, a credit card is handy for renting cars (since most agencies accept credit not debit cards), plus the chargeback protection that credit cards offer in cases of hotel/airline ticket mishaps etc. But this is the extent of thought I suggest you give to credit cards. Now, on to the traveling part!
Multimodal: Understand how intermodal connections work-- there may be times when the cheapest travel leaves a gap in your trip (say, a cheap flight into Ft Lauderdale but you want to see Miami (answer: Tri Rail train!); a cheap flight into Newark then out of JFK). If you become comfortable with connections like this, from air to train to bus, you'll have more flexibility.
Train/Bus: Speaking of intermodal, Amtrak Guest Rewards has award levels that are very competitive: Kansas City to Chicago to San Antonio to El Paso on a coach reward for 5500 miles (as in, 20% of what you need for an award flight). Similarly, Megabus has promotional fares of $1 or Free for its services that extend from KC as far as New York and Boston. With time (which you have!) and planning, you can score some of these. not that you should take the bus 3 days to Boston, but if you find a good price to Providence, then you can hit Boston and New York for a few bucks extra.
Cars: Rental cars can be either a convenient luxury or a necessity, and the biggest hirdle is finding a way to rent when you're under 25. Solution: USAA membership for free allows 18+ to rent from Hertz at competitive prices. Problem solved! Sign up at usaa.com
Evaluating offers: Marketing folks always have something to sell, but it's your responsibility to determine what is best for you. I remember reading an article on how to early free travel with "ordinary purchases you'd make anyway"-- including buying an SUV with cash, and refinancing a $400,000 home. Those aren't very ordinary for me (even as a working professional!) and I doubt they're ordinary for you. Be careful!
Planned travel: This is one of the best opportunities to take steps toward boosting mileage. If you need to fly to/from college, explore ways to maximize mileage..... but always keep an eye on the baseline cost. I've seen people create trips where the baseline ticket is $200, but they add in stops etc and increase mileage by 50% for a price of $300. On a cost per mile basis, they're exactly where they started. I suggest learn the nuances of pricing and fare rules, then learn how to stretch the fare.
Unplanned travel: Stay flexible and watch trends in the airline marketplace, then travel to where the low fares are. If Spirit starts new flights from Kansas City to Vegas, then $10 fares will most likely follow. If you're lucky, you may even see match by some majors, then you can work toward earning mileage for future travel. But keep your eye on the baseline cost; beware of opting for a $50 UA flight over a $10 Spirit flight in order to earn 500 miles.
Study abroad: As an incoming freshman, you have a great opportunity to plan study abroad once or twice during your school career. What better way to explore Europe than to have a homebase there for a semester, with Ryanair-type weekend trips. Even exchanges are available so you continue to pay tuition of your home school (with in-state discount, scholarships/grants etc applied), only that you're physically in another continent!
I hope this offers insight from someone who has been in very similar shoes!