Some points on the Witches to include in your essay:

They represent evil and cosmic disorder.
Their words are full of equivocation – evil works through deceit and false appearance.
They are perverse .
They create a sinister atmosphere.
They call themselves ‘the weird sisters’.
At the start of they play, they purposely seek out Macbeth in order to try to corrupt him.

It is Macbeth’s own choice to believe them – they cannot force him to do anything.

Some quotes:
‘Fair is foul, and foul is fair.’
(I.1)

‘Though his bark cannot be lost,
Yet it shall be tempst-tost.’
(I.3)

‘What are these,
So wither’d and so wild in their attire,
That look not like th’inhabitants o’ the earth,
And yet are on’t? Live you? Or are you aught
That man may question? You seem to understand me,
By each at once her choppy finger laying
Upon her skinny lips: you should be women,
And yet your beards forbid me to interpret
That you are so.’
(I.3)

‘If you can look into the seeds of time,
And say which grain will grow and which will not,
Speak then to me, who neither beg nor fear
Your favours nor your hate.’
(I.3)

‘And oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray’s
In deepest consequence.’
(I.3)

‘How now, you secret, black and midnight hags!’
(IV.1)

‘Macbeth! Macbeth! Macbeth! Beware Macduff;
Beware the Thane of Fife. Dismiss me. Enough.’
(IV.1)

‘Be bloody, bold and resolute; laugh to scorn
The power of man, for none of woman born
Shall harm Macbeth.’
(IV.1)

‘Macbeth shall never vanquish’d be until
Great Birnam wood to high Dunsinane hill
Shall come against him.’
(IV.1)