Être (to be) and avoir (to have) are the two most important irregular verbs in French — they're used constantly on their own and as the auxiliary for every compound tense. Être: je suis, tu es, il/elle est, nous sommes, vous êtes, ils/elles sont. Avoir: j'ai, tu as, il/elle a, nous avons, vous avez, ils/elles ont.
Both appear in fixed expressions where English would use a different verb entirely. Avoir is used for age (j'ai vingt ans, I am twenty — literally "I have twenty years"), hunger, thirst, and fear (avoir faim, avoir soif, avoir peur), where English always uses "to be."