It depends what you're trying to do.
The most important things I've learned have been the core skills and knowledge, such as statistics and neurology. But those are only useful if you're going in balls deep. If you're just aiming to have the broad overview, you're probably looking to get your hands on a first year text book. In broad terms, you want to understand:
- learning
- memory (particularly its fallibility)
- bias
- observer effects (just know that they exist and how they work ~ social psychology)
- nature vs. nurture
- individual differences
- development
- have some understanding of how your body is wired up to your brain, and how the brain is structured
- sensation/perception (know the difference, play with some illusions)
- clinical probably isn't that relevant to you unless you have a particular disorder or have an interest
- know the myths so you can dismiss the most prevalent pseudopsychology quickly