In some jurisdictions people can volunteer for jury duty. Often they are retirees who find it a good way to spend their free time. Some of them are quite knowledgeable as they come to the courthouse regularly to watch trials.
As for what jurors lawyers seek, it depends on the case. For example, lawyers defending members of minority groups often seek young college-educated jurors as they are presumed to be more liberal. Of course, the prosecutor can reject them but each side only gets a certain number of rejections that are not for cause.