Once the demo starts, you will see the Sign In Window:

This window contains slots for ten players ("mechanics"). You
can click "New" to create a new entry, or "Delete"
to free up a slot for use by someone else. Click "New" and enter
your name:

You will now see a brief outline of your mechanic's statistics:

Your "Skill Level" is a rating based on various
factors such as the number of cars you have repaired. As you progress
to higher skill levels, you'll gain access to more game features and cooler
cars.
"Total Time" tells how many hours of your life you've
dedicated to the greatest video game on earth. "Cars"
is the number of cars you have repaired, and "Cash" is
the amount of money you have in the game to purchase additional cars and
parts.
Click on your mechanic's name to start playing.