This is one of the biggest problems for most people. Having the dog walk by your side or slightly behind.

Before you go out on your walk, make sure the dog is nice and calm.

Firstly choose a side that you want the dog to walk on. Only use this side until the dog knows what is needed from them. You can train walking on the other side later on.

If the dog is on your left, hold the lead in your right hand. This gives you the chance to treat with the left hand and also use hand signals. 

Have the dog in the sit position. Take a step forward without the dog moving, then tap your left thigh with your hand and say "heel". This is you giving the dog the invitation to follow you. As he moves give him a treat with the left hand while he is behind you. Never give a treat for being in front. 

If they move on ahead and and the lead tightens,  hold the lead firmly in the right hand and stop. Don't move forward, give them a chance to slacken the line by moving towards you. When the line is slack again, take a couple steps backward and lure them back to the sit position at your left again  (facing the direction you are going ).

Repeat the moving off again. Every few paces treat the dog when they are slightly behind you. They will figure out that it is more positive to be beside you than forging out in front.

You may find you will be doing more stopping that walking at the start, but keep doing it. It will work.

 If your dog jumps up to get the treat while you are walking, raise the treat higher and keep walking until the stop jumping, the reward them.