They’re both right. Which one you use, though, depends upon the context of the sentence because they actually have two different meanings.

“All ready” is a phrase that is used to express preparedness. For example:

She was all ready to go to the beach.

“Already,” however, is an adverb that expresses time. It means, roughly, “by a certain time,” “previously,” or “even now.” For example:

Janice had already left when I arrived at the party.

“Already” can also be an expression of irritation:

Stop it, already!

They sound the same and are spelled so similarly, it’s easy to think one is a mistake. And this isn’t the only pair that trip up writers. There are plenty of similar sounding words that have different meanings, such as “than” and “then” or “it’s” and “its.” The only way to sort out the tangle is to know the meanings and uses. So, if you’re unsure, look it up in a dictionary or, better yet, a grammar book. Over time, you’ll make the distinction without any fuss.

They sound the same and are spelled so similarly, it’s easy to think one is a mistake. And this isn’t the only pair that trip up writers. There are plenty of similar sounding words that have different meanings, such as “than” and “then” or “it’s” and “its.” The only way to sort out the tangle is to know the meanings and uses. So, if you’re unsure, look it up in a dictionary or, better yet, a grammar book. Over time, you’ll make the distinction without any fuss.