Def do not need a car on campus, especially as freshman.
Here the page with the info about bus arrival times at various stops so you can track for convenience
http://www.transportation.umd.edu/nextbus.html
There is also a ridesharing option
https://umd.rideamigos.com/#/
And if you really want a car for a specific reason, zip car allows rentals to 18 year olds
https://www.zipcar.com
For transport to airport DURING BREAKS, UMD has FREE shuttle, so plan trips around BWI availability rather than Reagan/Dulles
(http://www.dots.umd.edu/trips.html)
Shuttle Service to/from BWI Airport
The Department of Transportation Services will operate free shuttle service to the Baltimore/Washington International Airport during Thanksgiving, Winter and Spring break.
Pickup Location:
All shuttles will leave from Cole Field House and pick up passengers from BWI Airport from Door 18 at the International Arrival terminal. This is the last door in the terminal.
Dates & Pickup Times:
Departure from Maryland to BWI Friday, March 16th (Leaves from Cole Student Activities Building)
5am, 7am, 9am, 11am, 1pm, 3pm, 5pm, 7pm
Departure from BWI to Maryland Sunday, March 25th (Returning to Cole Student Activities Building)
8am, 10am, 12pm, 2pm, 4pm, 6pm, 8pm, 10pm
We require no reservations or tickets, but passengers must show a University ID to ride.
If you have any questions please email dotsTrips@umd.edu. For Charter Operations call (301) 314-7271 and select option 1.
Transportation to BWI is also available by connecting to the Greenbelt Metro Station from College Park on the Green Metrorail Line and taking the B30 Express Metrobus to BWI Airport.
For airport info any other time, here is a page with options - note the mention of super shuttle being $28 one way
https://www2.cscamm.umd.edu/visitors/directions/airports.htm