To turn your dog into a guard dog you need to give him some basic training. This training will make him an effective guard dog.
Teach him to obey your commands: A stubborn or disobedient dog cannot be a guard dog. The training has to start by making him obey your commands first.
Let him socialize: Having a guard dog should not prevent friends and family from coming over. The dog needs to socialize to reduce his aggression and to be able to differentiate a visitor from an intruder.
Discourage him from barking unnecessarily: Guard dogs don’t just bark unnecessarily. So, when they start barking continuously, it will be obvious that an intruder is around.
Teach him how to attack intruders: Every guard dog should be able to attack intruders after giving them warning barks and they don’t back off. So, your dog needs training.
You may carry out the training yourself or take him to an expert. Just ensure your dog undergoes the basic guard dog training.