I am learning "le passé composé avec être et avoir" and how to change the endings of verbs that end in "er", "ir" and "re". What about the verb "nettoyer"? What would the auxiliary verb be? For example, how would you say "my sister cleaned her room"?
The auxiliary verb for the verb "nettoyer" is "avoir. You have to say "Ma sœur a nettoyé sa chambre.".
Here is something to start on the study of how to get the basic ending for the past participles of verbs : https://la-conjugaison.nouvelobs.com/regles/conjugaison/participe-passe-1.php. Then you'll have to learn the irregular forms and this reference will help you : https://la-conjugaison.nouvelobs.com/fle/les-participes-passes-irreguliers-47.php.