Select Play, followed by Start New Game. If you have played before, choose the save file that you want, or start a new one.
After the intro has played, follow on-screen guidance and listen to Amanda and her crew. The crew members are not mindless drones, but will seek out active objectives independently. You can issue them some direct orders by tapping their name on the left side of the screen. Later on, you will also be shown howto change crew member routes by dragging on the screen.
Try and maintain an awareness of where the Xenomorph is by using the cameras, and later, motion trackers. The crew can find shelter if you command them to hide, but remember, time is always running out.