Pay attention. Ever get admonished to do that by a teacher? Or a parent?
Writers need to pay attention to everything around them.
Listen to the way people talk to get clues about dialogue. If you are writing about something that happens in the deep South, in the US, then yes, you need to research expressions and listen to the cadence of the voices. If your story is set in Chicago, you need to listen to the way people talk in Chicago. If your hero travels to London, do some research on British accents.
Watch the way different people walk. You might need to describe how your main character walks. I noticed a woman walking gracefully in what appeared to be six-inch spike heels the other day. I was amazed at her grace and balance — and also filed that bit of knowledge for a future description.
Read newspapers or magazines either in print or online. The world around you is full of story ideas. I’ve had several writers recommend keeping a file of newspaper clippings. If that’s too old fashioned for you, keep a file on your laptop or desktop. Or your iPad. Read books, too. You don’t have to read just for research. Read for fun.
Ask questions. People love to talk about themselves and their hobbies. Or their work. Or anything that is of special interest to them. You never know when a character will turn out to be a scuba diver and you can find out more details about that from the nice guy you met at the coffee shop that you spotted reading Scuba Diving Magazine.
Challenge yourself. From memory, write a description of the doctor’s office where you sat and waited. Describe the other people who were waiting. Write a description of a building you pass every day on your way to work. The advantage here is that you can compare your memory to the reality.