The imperative in English

Use the infinitive to form the imperative.

affirmative sentences	negative sentences
Come here.	Don't come now.
Clean the bathroom.	Don't clean the living room.
Help your father.	Don't play on the computer.

Use the exclamation only when you want to make an exclamation, e.g. Stop! Help!

The imperative with let's:

affirmative sentences	negative sentences
Let's ask the teacher.    http://www.englisch-hilfen.de/en/grammar/imperativ.htm	Let's not ask the teacher.