Dog eats everything he can

I have a black labrador, and as many people know, the labrador lives and dies by his next meal.  What I am looking for is help on how to train him not to eat everything.  I live in a city and as you can imagine having a dog that thinks every thing on the ground is food, can not only be frusterating, but also scary when he eats some disgusting things that one could find on the street.   Can anyone help let me know if there is a goog trainning method I can try?

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