In formal settings, when you want to command or request someone to do something, you simply use the infinitive, ie. the dictionary form of the verb, followed by Sie. You should never leave out Sie.

 

 

Nehmen Sie eine Tablette drei Mal am Tag. Take one pill three times a day.
Gehen Sie zum Arzt. Go to the doctor.
Herr Müller, fahren Sie langsamer! Mr Müller, drive more slowly!

When you are being informal, you remove the -en from the infinitive, ie. the dictionary form of the verb, when you're talking to one person. But you add a -t when talking to more than one person.

 

Komm her, Peter! Come here, Peter!
Kommt her, Peter und Elke! Come here, Peter and Elke!