Very few modern cars have drain plugs on the fuel tanks otherwise fuel theft would be just so easy.
It may be possible to syphon most of the fuel via the filler neck. Some garages and breakdown services have a special pump for the job. Otherwise it may entail removing the fuel tank sender unit for access and a looksee inside.
In modern cars the only sure means of cleaning the tank is to remove the tank and have it professionally flushed.
Why do you need to empty the tank anyway??