There are several things you need to know: First, DO what you like. Second, do what YOU like. Third, do what you LIKE.
1) Do. The key is that your niche will involve doing something. It requires leaving your comfort one and acting. Make no mistake: finding your niche will not fall in your lap nor be presented on a platter. Instead, it will be discovered. You have to be about doing.
2) You. To be frank, finding your niche requires knowing YOURSELF first and foremost. While the undergrad years are fundamentally about learning, growth, and development, the graduate school years are about refining that self-awareness. There is too much at stake and too much demanded of you as a grad student to waste on excess. What do you like? What do you enjoy? What activities have meaning to you? What motivates you? However you answer these questions are a good starting point.
3) Like. Your niche will undoubtedly be discovered in something that you have enjoyed in the past. What is fun? What is rewarding? How do you like using your leisure time? Who do you enjoy working with? What causes do you champion?
So remember, you will find your niche by getting out and doing. what. you. like.