As soon as you start the game, you will be on a tram, unaware of your destination. You know nothing of the world around you and are unfamiliar of where you are. This is slowly revealed to you throughout the course of the level.
A Tram Ride
"Good morning, and welcome to the Black Mesa Transit System. This automated train is provided for the security and convenience of the Black Mesa Research Facility personnel."
These are the first words you will hear, and are only the start of your upcoming adventure. So you begin on a tram, unaware of where you are and what you have to do. The only thing you can do is sit back, listen, and look around Black Mesa as you head toward your destination.
Throughout the tram ride you'll see several interesting things. From huge robots to rockets, Black Mesa has it all. You cannot exit the tram at all and cannot use any sort of weapon. The only thing to do is look out the window. Several plot details will be revealed to you through the text and through the P.A. system, so observe carefully.
Sector C Test Labs and Control Facilities
When you finally reach your destination (Sector C Test Labs and Control Facilities), do what the P.A. system tells you to do. Wait for the security guard to come and open the tram's automated door for you. When you step out of the tram (make sure to search for personal belongings and take small children by the hand), follow the security guard to the large brown door.
Wait for him to open it, and then step on through. You will be taken to the next level. Look at you! The first level already completed.