It’s been years since I’ve been to DC. Metro tickets can be purchased online, you probably want a Metrorail Farecard. Best wishes, the March for Life is inspiring.
http://www.wmata.com/fares/purchase/passes.cfm
You can also check local Churches. Our dioceses in Illinois coordinated multi-bus trip to March for Life. In the early 90s it cost $90 for 17 hour bus ride to DC, attended the March, immediately followed by 17 hour return trip. Non-Catholics are welcome and encouraged to participate.
Go to Manassas or Fredericksburg and take the VRE in to Union Station. From there you can take the metro.