There are various ways to give a command or an instruction. Familiar commands are the most common as they use the Tú  (singular) and Vosotros (plural). 

In order to form the positive Tú command begin with the you or tú part of the verb.

AR verbs

 e.g. hablas.  Remove the S leaving – HABLA

ER verbs

e.g. comes.  Remove the S leaving – COME

 IR verbs

e.g. vives.  Remove the S  leaving VIVE

In order to form the negative Tú command begin with the you or tú part of the verb.

AR verbs  e.g. hablas.  AR verbs will take the ER ending ES – HABLES 

ER verbs  e.g. comes.  ER verbs will take AR verb endings AS – COMAS

IR verbs e.g. vives.  IR verbs will take the AR endings AS – VIVAS