On a walk my dog runs and barks at everything he sees. He's a real challenge. What can I do?

Beef up his basic training in the home environment, and once he's sitting and walking to heel on command, practice this in a park with few distractions. The aim is to get him listening to you. Also, teach him the "Look" command. The combination of better basic training and "Look" places you in control. For example, he's walking beside you and he tenses up because he sees a jogger in the distance. You get him to stop, sit, and "Look" at you, so his attention is on you and not the jogger. Hold his attention until the jogger has passed, at which point you reward him.