You can read the tutorial during the intro for instructions. Otherwise here's a list of controls.
W to inflate Jude (the higher her weight the faster the elevator goes up)
S to deflate Jude (the lower her weight the faster the elevator goes down)
A to send workers to the truck or let them off on the floor they need to be on
D to load the elevator with a box the worker is holding
Space to load a worker on to the elevator if you've already loaded their box onto the elevator
You can tell what floor the box needs to go to by the red number printed on the front of the box.
The more boxes are in the elevator, the heavier Jude needs to get to lift it.