How does Shakespheare create atmosphere in the opening act of Macbeth?

First off, you want to take note of the setting. It's misty, rainy, dark, and mysterious. In Shakespeare's day it didn't help much, but today's productions can benefit a lot by staging it that way. Then, the witches give the prophecy, which makes it clear that something bad will happen to Macbeth after he becomes King, which contrasts with his initial excitment at the idea.