gas gauge not working

I just bought this car a few days ago but found out gas gauge don't work after I purchased it. I filled it up the day I bought it and that's when I found it didn't work. Its stuck on empty or "R". I think it may be the fuel sensing unit but not sure. How much would on cost and how do I go about fixing it myself. I know the sensor is in the gas tank but I really need to get it fixed. The car almost died on me today because of no gas. Please help if possible.