The auxiliary verbs ESSERE (be) and AVERE (have) are part of so-called service verbs which support function in relation to other verbs supplementing or clarifying the meaning.
Generally, there are two basic functions of auxiliary verbs:
- help other verbs in the creation of compound tenses ( for indicative : present perfect , past perfect , etc…)
- form the passive.
Each type of verb needs only ONE auxiliary verb.
You can find some examples here: http://www.oilproject.org/lezione/verbo-essere-verbo-avere-verbi-ausiliari-grammatica-italiana-11088.html