This is an irregular verb. It follows the conjugation pattern of the verb pedir.
PresentPresente
El niño se desviste solo antes del baño.
The boy undresses by himself before his bath.
PreteritePretérito indefinido
Me desvestí rápido y me puse el pijama.
I undressed quickly and put on my pajamas.
Present SubjunctivePresente de subjuntivo
El médico quiere que el paciente se desvista.
The doctor wants the patient to get undressed.