It's usual for a jury to vote before deliberating because they want to know where they stand before discussing the case. It might be that all twelve of them agree on the verdict, and then it wouldn't be necessary for them to have to discuss the case. If the jury is split, they know they will probably have to take several days to go into the case in detail.
After the first round of voting, only one juror votes not guilty, and this angers some of the jurors who just want to get out of there because it's hot and they're tired. Three of the jurors give Juror Eight a hard time because he votes not guilty, but Juror Eight says he can't give a boy the death penalty without at least talking about it first. The other eleven jurors argue about it, but they finally decide to deliberate.