Direct negative tú commands are formed by changing the -as ending of the tú form in the present tense to -es and the -es ending of the tú form to -as.
for the verb fumar: fumas-->fumes
for the verb correr: corres-->corras

Let's look at some examples of proper use:

¡No fumes! - Don't smoke!
¡No corras! - Don't run!
¡No duermas! - Don't sleep!
¡No comas! - Don't eat!
¡No vuelvas! - Don't come back!
¡No pienses! - Don't think!

No te pares.
Don't stop.

No cierres la portezuela.
Don't close the door.

No le des la llave.
Don't give him the key.

No vengas mañana.
Don't come tomorrow.

¡No me hables de él!
Don't talk to me about him!