Why isn't phsycology a science?Watch
Obtain objective, empirical data, ie data that you can visually see and is a measure of cause and effect. Most of psychology is subjective and down to interpretation. There is no clear white or black answer.
Repeatability: for scientific experiments to be valid they must be repeatable. Most psychology experiments are in the form of case studies or such experiments where the participants have individual differences, therefore these cannot be repeated accurately.
Reproducibility: closely linked with the above, you cannot obtain the same results using different people or in a different time period.
Peer review: most scientific data has been peer reviewed and are published in scientific journals, articles etc. Whereas although some psychology data are in scientific publications not all are peer reviewed. You find most psychology related information online, in magazines etc, places where peer review doesn't occur.
There are more characteristics of science and pseudoscience (which psychology comes under), but I can't think of them right now