The imperative tense is used whenever making a command or demand.
The most common irregulars are:
-ar regular verbs
ligar ➜ ligue
Ligue agora!
Call now!
Coloque a panela no fogão.
Put the pan on the stove.
-er regular verbs
ter ➜ tenha
Tenha uma boa viajem!
Have a god trip!
Seja bem vindo!
Welcome!
-ir regular verbs
vir ➜ venha
Venha aqui!
Come here!
Vá embora!
Go away!
Many of these are often said in the TU form as well (read the post for more explanation):
ligar ➜ Me ligue! ➜ Call me!
colocar ➜ Coloque aqui! ➜ Put it here!
ter ➜ Tenha uma boa viajem! ➜ Have a good trip!
ser ➜ Seja bem vindo! ➜ Welcome!
vir ➜ Venha aqui! ➜ Come here!
ir ➜ Vá embora! ➜ Go away!