To avoid the mess around the stadium and the parking lot that is Deegan (I-87), I cross the Tappan and drive south on 9W to Irvington NY and take Metro North in from there. It's $4.75 to park (free on weekends). I've never had a problem finding parking spaces for weeknight and weekend games in Irvington. The direct train from there to Yankee Stadium takes about 30 minutes and is $8.50 Round-Trip (adult e-ticket that you can buy in advance or at the station). Metro North has special trains for the start of the game and adjusts the schedules to align with the end of the game. It's a 5 minute walk from the MTA station to the Stadium.
It's a hike from Maryland (what isn't); but the easiest way is the NJ Turnpike up to Woodbrigde and then take the Garden State north until it ends on I-87/NY Turnpike just before the Tappan. The exit for 9W is right after the bridge.
PS - As noted above by sledog, it is UN week. Avoid Manhattan, especially the East Side, at all costs.