I have a 2000 buick regal ls. My fuel tank gauge reads full when its not.How do I fix this?

I have a 2000 buick regal ls. My fuel tank gauge reads full when i start the car but its not always full. The only way for me to tell where my actual gas level is i have to turn the car on and dont start it up. The arm will move up and down i have to wait until it stops in order to know how much gas i actually have and i can be waiting for a while sometimes. I was told to replace the whole tank and dont want to if not necessary. How do i fix this?