To order someone around, use the present tense tu, vous, or nous forms. If the form ends in es, or as drop the final s.

parler tu parles
Parle ! speak
vous parlez Parlez ! speak
nous parlons Parlons ! let's speak
finir tu finis Finis ! finish
vous finissez Finissez ! finish
nous finissons Finissons ! let's finish
ir tu vas Va ! go
vous allez Allez ! go
nous allons Allons ! let's go

Chantez-moi une chanson.
Sing me a song!

Range ta chambre !
Tidy up your room!

Va faire les courses !
Go run the errands!

Ouvrons la fenêtre. Ça pue.
Let's open the window. It stinks!

Orders using the following verbs, however, are formed with subjunctive forms.

être to be Sois savoir to know Sache
    Soyez     Sachez
      Soyons     Sachons
  avoir to have Aie vouloir to want Veuille*
      Ayez     Veuillez*

Sois sage !
Be wise!

Aie de la pitie pour moi !
Have mercy on me!

*The vouloir forms are used to politely ask someone to do something. They aren't really orders.

Veuillez vous s'asseoir.
Please sit down.

When the verb is reflexive, use a disjuctive pronoun rather than a regular reflexive pronoun.

Réveillez-vous. Levez-vous. Taisez-vous.
Wake up.  Get yourself up. Shut up.

When a pronoun like y or en follows one of the verbs that drops the final s, don't drop the s

Go on.

Speak about it.

Lets go on

Tell someone not to do something by negating the verb in the customary way.

Ne sois pas en retard !
Don't be late.(formal)

Sois pas en retard !
Don't be late. (informal)