After talking to the Marshal, head over to talk to Marlow. This will start a mission where you play as Marlow looking for a distress signal on some kind of planetoid.
This mission is very simple, so follow the on-screen instructions and enjoy the ride. After you place the bomb and create a path for the rest of your crew, you'll discover the Alien spacecraft in the distance.
Enter the Derelict and discover the dead alien. Not long after, you'll find traces of the Nostromo crew nearby, along with some tracks.
Go look for the distress signal so you can shut it off to avoid others from discovering this place.
After disabling the distress signal...
return to the main room with the dead alien and enter the winch.
Down in the caverns, watch as one of your crew gets attacked by a Facehugger.