What do we discover about the witches in the opening scene of Macbeth?
In Act I, Scene I of Shakespeare's Macbeth, we learn that at least two of the witches have "familiars," which are spirits that appear as animals. The first witch calls to Graymalkin, a cat-like familiar. The second witch calls to Paddock, who is like a toad. The third witch's familiar is not described.
We learn that they plan to meet again, in a field, after the battle is over. We can infer that they have predictive powers at this point, because the third witch proclaims "that will be ere the set of sun" in reference to when the battle will end.
We also learn that they are planning to meet Macbeth after the battle. Later on in the play, the witch's prophesy about Macbeth's future.
This is all we learn about them in the opening scene. The witches appear again in Act I, Scene III, where we learn more about their appearance. Banquo speaks to them, saying,
"How far is’t called to Forres?—What are theseSo withered and so wild in their attire,That look not like th' inhabitants o' th' Earth,And yet are on ’t?—Live you? Or are you aughtThat man may question? You seem to understand me,By each at once her choppy finger layingUpon her skinny lips. You should be women,And yet your beards forbid me to interpretThat you are so."
"But in a sieve I’ll thither sail, And like a rat without a tail, I’ll do, I’ll do, and I’ll do."