Short 1-minute video on how to fill a barrel for barrel-aged beer.
If the barrel has sat for a while, the staves may have dehydrated and you'll need to fill with water to check for leaks. Hot water can help sanitize as well.
I purge my barrel with CO2 before filling, but you could also add a bit of sugar/wort and put an airlock on it to have the yeast scrub any oxygen.