What information is revealed about the crime to the reader before the first vote?

We don't know a lot about the defendant at first. It's only later on, as the play progresses, that we come to learn more about him through the reactions of the jury members to the evidence that has been presented to them.
Before the first vote, we discover that the defendant purchased a switchblade which he claimed to have lost through a hole in his pocket. This seems rather suspicious, to say the least. The defendant has been charged with stabbing his father to death, and so his admission that he bought a switchblade would appear to point towards his guilt.
That being the case, it's no surprise that the first vote ends with all but one of the jurors—juror number 8—voting guilty. The judge has required a unanimous verdict, and so because there's a hold-out on the jury there'll need to be a full-blown discussion of the case.