Learn Italian: Noi non siamo pronti.

You can usually leave noi out! In Italian, the verb itself tells you who is doing the action. Since siamo only ever means "we are", saying non siamo pronti is perfectly natural and actually more common, unless you want to emphasize we instead of someone else.

Why is the word for "not" (non) placed before the verb?

In Italian, the negative word non almost always goes directly before the verb it negates. While in English we say "we are not", the Italian structure is always literally "we not are" (noi non siamo).

Why does this sentence use pronti instead of pronto?

Adjectives in Italian have to match the person or thing they describe in both gender and number (singular or plural). Because noi (we) is plural, the adjective pronto (ready) changes its ending to the plural form pronti.

What if "we" is a group of only women? Would it still be pronti?

No, if the group is entirely female, the adjective takes the feminine plural ending -e, making it pronte. You use the masculine plural pronti if the group is all male, or a mix of men and women.