This will be unlocked in episode 1. I would suggest just following the walkthrough here on TA:Back to the Future: The Game - 30th Anniversary Edition Walkthrough
Here is the description coming from the gamers who created the walkthrough (Credit to them of course):
Now to serve this subpoena. Leave the park and cross the street off in the distance. You should be back in the sidewalk by the Law Offices sign. Continue down the street to the barbershop and talk to Kid Tannen. Agree to shine his shoes and mention you're looking for Arthur McFly. Ask "Isn't that Artie's hat?" and then ask for some of the peanuts Kid is keeping in it. When he agrees, say "What in the hell is that!!?" to snatch the hat and escape from Tannen's gang with Emmett's help. Return to the park and use the hat on Einstein to track Arthur down.
After arriving at a familiar apartment, we can immediately head to the right and back to the town square. Now that Kid is back outside the barbershop, go over to him until you're chased onto the roof of the gazebo. Use the tape recorder on Kid, then interact with Einstein to get down once you've got the recording. Head back to the police station and continue past it to the left in order to return to Arthur's. Use the buzzer, use the tape recorder on Arthur, then use the subpoena on Arthur.