Don't panic. You don't have to put on a wig and start following people around. I'm not advocating that you act like you're in a film noir. You shouldn't sit around and just observe your group of friends and family; this is creepy, and it isn't going to get you very far. It's best if you observe life across a variety of social classes and economic levels. Before you buy biker leather and attempt to infiltrate a gang, however, try basic research.
It's a skill that every writer has to have, and when you're researching the human condition it's actually pretty fun (not like looking up the history of writing utensils or silverware). For starters, try watching reality TV. This is sort of an extreme version of normal human behavior -- think of it as shock therapy. For example, I make no secret of the fact that I watch a lot of true crime shows. Since I write mysteries, this makes sense for me. If you're writing about romance in your books, see if you can stomach watching a dating show like The Bachelor. A lot of feelings get explored and exposed on reality TV, and it's much more helpful than it sounds.
Look at past historical figures to learn more about human nature. Who interests you? Look them up in the encyclopedia, start checking names that strike your fancy, and read bios. History can show us a whole lot about human nature, the good and the bad.
Observe life in all its forms, from the flowery historical accounts to the made-for-TV histrionics, and expose yourself to lots of different attitudes and personalities. Studying the humans is going to make you a better writer in the end, and if you have fun with it you'll learn a whole lot that can help you in your work.