how to show guide about gameplay at first game launch

1) You can use PlayerPrefs and check if some specific key exists. if it exist then it is not first time. if it is first time then save the key and show demo. 2) You can not delete a scene from a build. You can skip it but it will still be there in your game.
3) This extra scene will not affect performance but size of the build only.

Thanks so much @Yash, but thing that i forget to say is that my game is for andorid... Is there this like solution for it?