I'm not sure if you have to put these in the past tense because you're not using some of them the right way with the past tense. Most of the verbs that end in IR stay the same when you conjugate them using the Yo form. So....
Decir - Yo voy a decir algo muy importante a Maria / I am going to tell Maria something very important
Pedir - Yo voy a pedir una hamburguesa y un refresco / I am going to ask for a hamburger and a drink
Repetir - Puedes repetir eso por favor? / Could you please repeat that again?
(this stayed the same because there is a conjugated verb in front of the word already and you don't conjugate two verbs in a row usually)
Reir - Nosotros reimos cuando me callé / We laughed when I fell
Your sentence for Sonreir is good! Wouldn't change it.
Despedir - Despedimos a mi padre cuando se fue al trabajo / We said bye to my dad when you left for work