How do you retrain a dog that learned to go potty inside?

i have a 5 year old maltese poodle that we got when he was a puppy. unfortunately, we didn't get him in puppy school soon enough and thought we could train him on our own but never got around to it. needless to say, lack of training trained him to go wherever he wants in the house. we've put up with it for too long and want to fix the problem but don't know the best way to do it or if its even possible to retrain a dog that's been doing it this way for so long. any ideas? thanks!