Hmm, beer still in it you say? Drink it!
As for cleaning. Rinse it out really good with hot water a few times. Then put in a warm water solution with oxyclean and let it sit overnight. Shake it a few times to make sure all surfaces are exposed. Drain and rinse very well the next day.
Replacing the seals is probably a good idea. Sanitize before usage with a Star san solution. As for filling, rack into the keg just like you would a bottling bucket. Close up the lid and hit it with 10 psi of CO2. Purge it a few times with relief valve ring (if pepsi ball lock keg, depress gas "in" if coke pin lock keg). This purges oxygen out. Then ramp to 30 psi to seat the seals, spray some star san solution around fittings to check for leaks. Now your ready to place in fridge and carbonate (see threads on proper carbonation temps/CO2 pressures).