Depending on what's been in it? If fuel or non-solid-forming liquid - put 20lts or more of hot water & ample detergent (cup of washing powder is good), swish around, let settle a few times & empty. Put in another few lts of water but leave in. Ensure that the lid/cap is off for ventilation and with a fire extinguisher handy & good PPE get stuck into it with a grinder and 1mm cutting disc!
DON'T try this if it stored a solid-forming liquid (some oils, grease, paint etc) as the water won't remove the solids and they may become explosive.
Nibblers are also handy, but a grinder will get the job done smoother & faster.