Don't just run out to the petstore and get a new bird. You can't just put a new bird in your existing bird's cage and expect them to get along. If you decide to get another bird the new bird needs to be set up in it's own cage in a separate room at least 30 feet from the existing bird for a period of no less than 30 days (quarantine.) That way if the new bird is ill it won't infect the old bird.
After the quarantine period is over you can place both cages in the same room. You can gradually start letting the birds out at the same time while supervising their behavior. At the same time you can start moving their cages closer together, until eventually they are side by side.
The birds may let you know that they are ready to share a cage by doing so on their own, or you may be able to move the process along.
Think about it this way. How would you feel if you woke up one day and there was a stranger living in your house? You have no idea who they are or where they came from, but there they are, and they won't leave.