The verb avoir in French is not only “to have”, but also a whole bunch of expressions that sound strange if you translate them word for word. Here are a few examples that are sure to come in handy.
First, age in French is have. Instead of “I am 20 years old” you should say J'ai 20 ans - “I have 20 years.” It's always said this way, even if one wants to say “I am young” or “I am old” because it's just a rule.
Secondly, feelings and states of mind are also “have”. The French don't say “I am hungry”, they say J'ai faim - “I have hunger”. Similarly with thirst (J'ai soif - I am thirsty), cold (J'ai froid - I am cold), heat (J'ai chaud - I am hot) and even fear (J'ai peur - I am afraid).
In addition, avoir is used in expressions that denote mental ability. For example, if a person is right, the French will say Il a raison - “He has the rightness”. And if he is wrong - Il a tort (“He has a mistake”).
Another useful expression is avoir besoin de, which means “to need”. For example, if you need a telephone, you would say J'ai besoin d'un téléphone.
And, of course, you can't do without avoir when talking about luck. J'ai de la chance is not “I am lucky”, but “I have luck”.
It is better not to translate these expressions word for word, but to memorize them in their entirety, because they occur in the language all the time.