Hello, welcome to AHS!
I would say that you could start with A BEST, I am..., or RAINBOW. Maybe even Duty. But like Phantoma said, you should list the songs you've heard already.
MY STORY is a great album too, but in my opinion it seems like an album for, not really what I'd call "advanced" Ayu fans, but an album that you should listen to after you've heard stuff from albums like I've previously mentioned.