The phrase "all ready" means completely ready, or prepared. You are all, completely, ready. (My son is all ready for his big date). (The car is clean and gassed up all ready for prom night.)
The word "already" can be used to mean "previously" (He already left to pick up his date), or "so soon" (I can't believe he's already old enough to go to prom.)
Keep them apart by remembering that if you're completely ready, you are all ready, while if you're worried about the passage of time, it's already later than you think!