Start with short walks, perhaps around your house or yard. Reward for desireable behavior. If he starts tugging on the leash or chewing on it, stop and say "NO", put him in a sit, and then continue on. The second he is walking nicely, praise and reward. The second he starts being "bad" on the leash, stop and wait until he is ready to walk nicely.

Once he learns he gets treats for doing it YOUR way, he'll become MUCH better!