func prepare(for segue: UIStoryboardSegue, sender: Any?)

There are two methods to select first indexpath in collectionview like below

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

let indexPath = self.collectionView!.indexPathsForSelectedItems?.first

let indexPaths = self.collectionView!.indexPathsForSelectedItems!

let indexPath = indexPaths[0] as NSIndexPath!

}