Grammatical gender has nothing to do with biological sex. They're just labels given to noun classes. In Italian, "banana" just happens to belong to the noun class labeled "feminine."
A fairly easy way to tell what class a noun belongs to in Italian is to look at the last letter. Most of the time (there are some exceptions) this is a good rule of thumb:
-o - singular masculine
-i - plural masculine
-a - singular feminine
-e - plural feminine
Please note that this only applies to nouns and the adjectives that must agree with them. This has nothing to do with how verbs conjugate.