Socialising your dog is an essential process and one which should start as soon as you get him home. Introductions to as much various people, surroundings and activities as you possibly can early in life will help him build confidence. It will also help you feel more confident in managing in these new situations.
Be sure that you're as relaxed and comfortable as you possibly can in new or difficult scenarios in order to not resonate any negative energy to him.
Remember that they may be looking to you for help and guidance, as you are their pack leader. Every action you make will undoubtedly be transmitted to him. For instance, if another dog approaches while you are out walking and you tense up or run away, your puppy may see the other dog as a threat and respond accordingly.
If this pattern were to occur regularly then he would begin to expect this and pre-empt you by reacting aggressively immediately upon sensing another dog, it is in this way that bad habits are formed.