With aggression problems, prevention is definitely better than cure. Prevention in the form of socialisation of puppies is essential. They must be prepared for situations where they may feel threatened in the future. Owners also need to educate and make themselves aware of their dog's own body language and not ignore the warning indications their dog is showing.
If your dog is already showing aggressive behaviour towards humans or dogs, then a professional behaviourist will use various techniques depending on the type and cause of the aggression involved, to solve or improve the problem.
In the meantime we would recommend you consider using a muzzle on your dog in situations where he is likely to be aggressive - especially in public because of the legal implications involved. This is to protect your dog from getting into trouble, as much as to protect other dogs or people from being bitten. Please ask your vet or a behaviourist to show you how to safely introduce your dog to wearing a muzzle. It would also be very advisable to obtain third party insurance against your dog causing an injury.
If you are worried about your dog's aggressive behaviour, please ask your vet to refer you to a behaviourist for advice.