After you have filled your keg with beer you would apply some pressure, CO2 to seal the lid, purge out oxygen and make sure there's no leaks. If that's ok, then your beer is going to take some time to carbonate.
I force carbonate my beer, meaning it sits in my beer cooler under pressure, 12psi at the regulator, for a couple weeks.
You could also add priming sugar to your keg to carbonate it, still it's going to take time to carbonate.
If you have done either of these methods and your beer is flat, you have a leak somewhere. You'll need to track that down by apply pressure to the keg and spray soapy water on any joint. Look around the keg lid, pressure relief, gas/beer posts and anything connected to the keg.