How is Macbeth presented in Act I of Shakespeare's Macbeth?

In Act I of Shakespeare's Macbeth, Macbeth is depicted as a brave and ruthless warrior. He is also an ambitious man who is affected by the preternatural world, his cupidity, and his determined wife.

In Scene 2 of Act I (Macbeth is not in Scene 1), a bleeding captain comes into a camp in Forres and reports that the traitorous Macdonwald has been slain by the brave Macbeth, who "disdained fortune" and ruthlessly split the traitor from the navel to the jaw.

In Scene 3, as he is accompanied by the Scottish nobleman Banquo, Macbeth enters a heath near Forres. There, he is both frightened and seduced by the three witches that he and Banquo encounter. The witches' natures are rather menacing, as are their messages to Macbeth and Banquo. They first greet him as Thane of Glamis, then Thane of Cawdor, and finally as King. Macbeth then asks the "three sisters" to explain what they mean by calling him these three different titles. Banquo 
urges Macbeth to be careful because such preternatural creatures as these witches sometimes tell men truths that later trick them into dangerous and destructive circumstances. So, it is with a sense of foreboding, also, that Macbeth thinks to himself,

This supernatural soliciting
Cannot be ill, cannot be good (Act I, Scene 3, lines 134-135).

Further, Macbeth feels that imaginings and reality are blurred, and he is also confused about good and evil.

My thought, whose murder yet is but fantastical,

Shakes so my single state of man

That function is smothered in surmise,

And nothing is but what is not (Act I, Scene 3, lines 142-145).

While he has earned the title of Thane of Cawdor through his bravery, Macbeth finds himself still pondering the witches' having called him king. He knows that to be king, Duncan must be murdered, and he finds his imaginings more dangerous than the crises he just faced in battle. Now, his ambition causes him to equalize reality and fantasy.

If chance will have me King, why, change may crown me,
Without my stir (Act I, Scene 3, lines 147-148).

In Scene 4, King Duncan names his son Malcolm as his successor. Now that Macbeth has his sights set upon being king, he perceives Malcolm as an impediment to this goal.

In Scene 5, Macbeth returns to his castle, and Lady Macbeth talks with him, urging him to carry out a plan to murder King Duncan so he can be king. First, they must "bear welcome in the eye" (Act I, Scene 4, line 63); that is, be deceptive around their guests.

In Scene 7 (Macbeth is not in Scene 6), Macbeth engages in a soliloquy that reveals his inner conflicts. For instance, he wishes that the assassination of Duncan could be done without consequences. Then, too, he recognizes the goodness of the king—who is also his kinsman—and Macbeth wrestles with his conscience. In the end, however, his "vaulting ambition" and Lady Macbeth's ridicule sway Macbeth to go through with his plan to commit regicide.

In Act I, Macbeth is presented as a very ambitious man who is willing to be ruthless and commit regicide. He subjugates his better nature to his "vaulting ambition" and his wife, who he will not allow to challenge his manhood.