I am going to assume you mean the "informal" command. These are tú commands. You would use these commands with friends, family, and informal peers (classmates, for example).
I added a table with columns for affirmative (do it) and negative (don't do it) commands.
Affirmative <em>tú </em>commands are formed by using the third person singular form of the conjugated <u>regular</u>verb. There are only eight irregular verb--see table.
Negative <em>tú </em>commands are formed by doing the following steps:
See table for irregular verbs -- They're color coded
For regular negative <em>tú </em>commands verbs, put the regular verb into the <em>yo </em>form, (i.e. <em>hablo, como, escribo</em>) [This captures stem changing verbs and other spelling changes
Remove the -o/-oy (i.e. <em>habl, com, escrib)</em>
Add "<em>No</em>" before the verb and add the "opposite" ending (i.e. <em>hables, comas, escribas</em>)
Explanation:
The above briefly explains how to form informal commands.