"Let's go" teaches your dog to walk nicely on the leash without pulling.  It is not the same as heeling.  You need to start in your yard with the dog on the leash.  Start walking and when the dog goes ahead of you, immediately turn and walk in the opposite direction saying, "Let's go!"  The dog will bound after you and immediately run out ahead of you again.  Simply turn in the opposite direction and repeat, "Let's go!"

What you are teaching the dog is to follow your lead.  You are the leader.  It's the dog's job to pay attention to where you are going.  It's not your job to guide the dog, or walk around the dog, or follow the dog while he wanders aimlessly sniffing.  Your dog should walk next to your side without pulling on the leash.

Please note that this is different from the heeling exercise where the dog needs to sit every time you stop and remain in a precise position at your leg looking up at you as you walk.  With the let's go exercise, the dog is allowed to look around and be fairly liberal as to how close he is to your side, however, he should never be out ahead of you, and he does not need to sit when you stop.

After a few times you can try doing this with distraction.  Your front yard will have many more distractions such as children playing nearby, cars driving by, other dogs and cats, etc.  Repeat the same lessons until your dog ignores everything else and pays attention to where you are going.

Now you are ready to go to a park and repeat the same process.  There are many more distractions at parks such as ducks near a lake and squirrels, etc.  Repeat, repeat, repeat -- never allowing the dog to pull on the leash.  The leash should be slack at all times.

Next, you are ready to walk down the street on the sidewalk.  You might not get very far the first few times, because every time the dog goes out ahead of you, you must turn around and go in the opposite direction.  You might begin to feel like a metal duck target in a shooting gallery but stick with it and reap the rewards.  Your dog wants to go forward so it won't take long for him to figure out that the way forward is not to pull on the leash and to keep close by your side.  If you have done your work at home and in parks, walking with your dog on a sidewalk without him pulling will come fairly quickly.

And, remember to praise the dog and reward with a treat here and there along the way.  Make walking fun.  After all, wasn't taking pleasant walks together one of the reasons why you wanted a dog in your life?