The imperative gives commands or instructions and exists only in the tu, nous, and vous forms — there's no je, il/elle, or ils/elles imperative, since you can't command yourself or a third party directly. It's formed from the present tense, simply by dropping the subject pronoun: tu parles → parle!, nous parlons → parlons!, vous parlez → parlez!
For -er verbs (and verbs like ouvrir that conjugate like them), the tu form additionally drops its final -s: tu parles → parle, not parles. Object and reflexive pronouns move after the verb and attach with a hyphen in a positive command (lève-toi!, get up!), but stay in front in a negative one (ne te lève pas!, don't get up!).