How do I get rid of all of these gnats in my room?

They just fly around all day and it's so annoying! I keep killing them but somehow there's still a lot. They like to land on the two mirrors in my room. This happened a month ago but they were in the kitchen too. They were surrounding my trashcan and the trashcan in the kitchen. There was some rotten fruit in each of them. After we threw the trash out they were gone. They're barely starting to come back again. There was one time in December when I woke up and there were these giant gnats, or whatever they were, just on the floor dead! So I had to vaccum them up. How can I get rid of these pests?