I did half the PPL exams with only a half-hour trial lesson behind me.
Given that most schools will not let you take your PPL skill test without having passed all the exams and many schools will mandate certain exam passess before certain stages of training (i.e. Air Law before first solo etc), the more that you can get done before you start training, the easier the process is.
If you are doing a PPL full time, you are less likely to have the time to get through the subjects adequately so doing a few beforehand means less pressure whilst training, especially if you're going abroad for the training.
Air Law, Meteorology and Human Factors are three that can easily be studied without any previous aviation experience.
Cheers
Whirls