I have killed duke venim in an early mission and now I cant become redoran hortator. This is for the xbox, any help would be appreciated!

Unfortunately I don't think there's anything you can do. :/ It's an inherent bug in the game that if you do the Redoran kill Venim quest before the main quest you can't get Hortator. I think it works vice versa as well. You could, perhaps, by using the console, but since you're on XBox that's not an option... ><

Basically, reload a save game from before you did the quest or start again I think...