You need to remove the fuel tank, drain all the gas out, clean it thoroughly, coat the inside to prevent corrosion and probably replace the fuel lines.
Modern gas is terrible when it sits around, and can ruin an engine if started with it. I had to learn this the hard way.
There are firms that provide tank cleaning service and coating. It will probably cost you $200.
Owner of Loud Pedal Performance in Mocksville, NC, specializing in automotive restoration and customization.
My current Mustang: 2005 V6 MT - Pypes X true dual with GT mufflers - GT suspension and wheels - Prothane shifter bushings - GT Fogs