As other posters have said, the audience doesn't actually meet Macbeth until Act I, Scene III.  However, because of the groundwork that Shakespeare lays beforehand, we are given a good indication of Macbeth, his characteristics, and what he is in for well before he appears on stage.

The play opens with a brief scene en media res.  Three witches are just finishing a meeting and discussing when they will meet again.  In beginning the play in this way, Shakespeare establishes an eerie, sinister mood.  When the Weird Sisters they reveal that they next time they will meet will be "[u]pon the heath/ There to meet with Macbeth" they let the audience know that Macbeth will be part of their next sinister gathering (7-8).  At this point the viewer doesn't know if Macbeth is already aligned with the witches or is innocent of any wrongdoing, but given that the play is entitled "The Tragedy of Macbeth," the audience can guess that even if he is innocent, he will not be for much longer.

Shakespeare builds ambiguity through the continuation of the audience's introduction to Macbeth in Act I, Scene II.  In this scene, a wounded captain is returning from the battle to quell both the rebellion and the invasion of Norway.  Upon meeting with King Duncan, the captain further illuminates Macbeth's character:

For brave Macbeth—well he deserves that name—
Disdaining fortune, with his brandished steel,
Which smoked with bloody execution,
Like valor’s minion carved out his passage
Till he faced the slave;
Which ne'er shook hands, nor bade farewell to him,
Till he unseamed him from the nave to th' chops,
And fixed his head upon our battlements. (16-23)

In Shakespeare's Macbeth, the character Macbeth actually isn't physically introduced until scene 3.  In scene 3, he and Banquo simply walk in on the three witches just as they finish a charm.  Macbeth comments on the weather using the same words spoken by the witches at the close of scene 1 (which of course in some way, depending on one's interpretaion, connects Macbeth to the witches):  "So foul and fair a day I have not seen."  Banquo then notices the witches, and the scene takes its course.

Macbeth's name is mentioned in scene 1, by the witches, when they reveal that the three will meet again when they meet with Macbeth, and again in scene 2 when King Duncan gives Macbeth credit for the military victory over Cawdor.  These mentions of the character prepare the audience for Macbeth's appearance in scene 3.