The main thing you need when you read any of Austen's novels in general – and this one in particular – is a fine-toothed comb. OK, not literally, obviously. Although who knows? Maybe you brush your hair while you read, and it's really not any of our business. Metaphorically, though, you certainly need a comb because what makes Pride and Prejudice so super (super-awesome, super-hilarious, super-duper you name it) is also the easiest thing to miss: nuance. Every tiny turn of phrase, every slightly raised narratorial eyebrow, every throwaway aside to the reader – these are crucial to both style and substance here.
Other than that, there are a few of the usual suspects when it comes to reading something written long ago and far away: 1) getting used to the different quantitative systems – money and distances are really different when you're pre-industrial revolution and pre-railway travel; and 2) getting used to the different social norms and values – back in the day, an unmarried woman is considered a parasitic waste of life, basically, so the drive to get married is strong, urgent. It's much more loaded than nowadays that it's better thought of as a woman's job than her personal life.