I bought a very cool book at the shop of the Royal Opera called The Art of Auditioning. The author is English, but UK, US and continental competitions are listed, together with the upper age limits. The German and Swiss agents are listed. The author is Anthony Legge, Head of music at the English National Opera.
It's all here: what to bring, how to act, what to sing and how to prepare. There is interesting advice about checking the edition of the arias you are singing to make sure they are the same as the score. How would you ever think of that?
I totally had a 'tude about this sort of thing. I adored to perform in front of an audience and hated, loathed, despised, detested any kind of auditioning. I was wildly furious the entire time, ready to strangle the lot. I needed attitude adjustment training.
He talks about the cadenza books and says to use them. I'll explain. There are books with written out ornaments you can perform with the arias. He recommnds Ricci. Sometimes there are several versions for the same aria so you can choose cadenzas that suit your voice.
He has a Fach list of his own with roles. He says to be a "dramatic mezzo", as I now claim to have been, you need "striking physical beauty", so I guess I didn't really qualify.
If you feel absolutely clueless about this subject, this book might help.