People get scared for lots of reasons. Some people get scared when they see snakes and spiders. They might be afraid that they could be bitten. Some people get scared by heights. They might be afraid that they could fall and get hurt. Some people are afraid of balloons. Maybe a balloon popped right in their face when they were young, and they don't want that to happen again.
You might get scared in the dark for some of these same reasons. You might be afraid that you will trip and fall and get hurt because you can't see where you are going. You might be afraid that something you can't see will jump out and startle you. It might just be fear of the unknown. It could be a combination of things.
You might try this. Go into a room where the lights are on. Clean the floors so you can't trip. Close the door and check the whole room so you know that you are the only living thing in the room. Now turn off the lights and sit on the floor. Do you feel fine, or do you feel scared? If you feel scared, ask yourself why. If you need to, turn the lights back on and look around. You know there is nothing in the room that can hurt you or startle you, so why are you scared? If you think about it, you might be able to understand what is making you feel scared in the dark.
If you sit in the dark long enough, your eyes will adjust. Unless the room is dark like a cave, there will be a little light. After your eyes adjust, you might be surprised at how well you can see in the dark.