Good lighting for both your computer and for your notes and research. A flat surface where you can spread out notes, books, and other materials. A place to file research notes, articles, and other information. A place to store supplies where they are readily accessible. A space for your most useful reference books.
Some people prefer to work in privacy, with a door they can close between themselves and the world.
Whether you work best from a closet, an attic, or the middle of the kitchen, the key is to make sure that others respect your space and will not disturb it or move your materials. Clear your schedule. Lack of time is one of the biggest obstacles for every writer. You'll be amazed at how many tasks, demands, and interruptions crop up to prevent you from writing. Don't expect anyone else to cut you any slack.