These are easy to mix up when writing because they are pronounced the same way even though all ready is 2 words and already is 1 word.
All ready is a phrase that means absolutely, totally, completely ready :). For example: I studied a lot last night. I am all ready to take that test now!
Already is an adverb that usually has 2 meanings. First, it can mean that something happened sooner than you expected. For example: I can’t believe you already bought all of your Christmas presents! It is only July!
It can also mean that something happened before the moment you are speaking. For example: I can’t believe you already saw that movie! I thought we were going to watch it together.
So, how can you prevent mixing them up? I like to think that if I am ‘all ready’ I don’t have to rush anymore because my work is done, so I have time to write 2 words. Another tip is that already is usually used with the present perfect tense (She has already eaten–it happened at a non-specific time before right now), but all ready is often used with either the simple present or simple past (She was all ready for her trip, so she left. I am all ready for the hug you promised me).
I hope you found this tip helpful! Good luck :).