Use the same command e.g. 'Sit' in the same tone of voice each time.
Use the word and put the dog in the position.
Praise the dog when it does well (with affection not food).
Use different tones for different commands.
Don't shout at your dog: it doesn't understand English.
Tell the dog off (don't hit it) when it does wrong.
Repetition is the key. The dog will gradually learn what to do.
Always use a lead when in public.
Dogs must be used to traffic, other dogs and people.