Macbeth Response

I remember reading Macbeth in high school, but never quite understood it. It was kind of like poetry. Like a different language. We also read Romeo and Juliet so I think part of the reason it was hard to understand is because I would get the two stories mixed up. Lets hope this time I can get it right.
The very beginning of Macbeth was always unclear to me.  I never understood why there were three witches chanting spells and such. Re-reading it again and having seen the movie in high school still did not help. I just do not really see the connection to Macbeth.  The very first line asks when the three of the witches are going to meet again.(Shakespeare 2) I would rather know why they are meeting now than when they are going to meet again.  They also mention how they will meet Macbeth on the heath. (Shakespeare 2)
This passage, to me, would make more sense if we had background knowledge about the witches, Macbeth, and locations and everything.  In my book there were a few pages before the actual story begins describing the time period, location, etc. of when Macbeth was written, but not when it takes place. I am hoping that next class we can review what we read as we did with "Oysters". I don't think I was the only one who got a better understanding of poetry after we reviewed it. I hope that we can do the same with this so we can understand the story better. Right now I'm kind of lost.