Already and All Ready are quite indistinguishable in speech. However, the two words have distinct meaning and uses when it comes to writing.

Already is an adverb which means previously or prior to some specified time.
When we reached station, we found that the train had already left.
When I came, the show had already started.

It could also mean so early or so soon.
Is it dark already?
Have we reached already?

All ready is a phrase which means completely ready or well prepared
We are all ready to leave for our vacation.
They are all ready to set out on this quest.