Formal:
* Nous sommes prêts. (This is the most literal and formal translation.)
Informal:
* On est prêts. (This is a more casual way to say it, using the pronoun "on".)
* On y est! (This is a very informal expression meaning "we're there!" or "we're ready!")
Other options:
* Nous sommes parés. (This is a slightly more formal and literary way to say "we are ready.")
* On est fin prêts. (This is a more informal way to say "we are fully ready.")
The best choice for you will depend on the specific situation.