In act 1 Scene 1 what effects is Shakepsear creating by beining the play with this scene?

As fancy and important as Shakespeare might be (and he is), his work was still for an audience. In the same way that a television show begins with a "teaser" to pull an audience in, so does Shakespeare start his tragedy of Macbeth, with its dense and dark themes, with a weird little scene of witches. It's a great way to get us interested in what's to come. This is especially true because they are forecasting what will come to pass - "fair is foul, and foul is fair" - thereby creating in an audience's mind questions about what the witches are suggesting.