Who do you go after first?

Fleur, Of Course!

Deciding to 'Go Big or Go Home', as the Muggles would say, you turn a corner heading towards the Hogwarts Owlery, a plan already forming in your mind. If you could pull this off, you could have a direct route to getting Sirius a trial! That you can get a lot of extra 'benefits' out of is just the 'icing on the cake'.

When you get there, you pull out a piece of parchment and a quill and start writing.

'Dear Ms. Delacour, I have very important information about the upcoming First Task and I think I may have something that will help. Meet me in a clearing in the Forbidden Forest behind the Gamekeeper's Hut and we can talk.'

You intentionally don't sign it so Fleur hopefully thinks it's just some secret admirer wanting her attention, you give just enough information in the letter so that she has **** to come see what you know. You included the bit about the 'help' to hopefully give you an excuse to claim her.