I have an idea let's say something like CamScanner How do I go about building application? Can I see the YouTube videos for iOS and Android? Is there any specific requirement sets or guidelines followed in companies like fb, google, Twitter? Should I develop iOS and Android separately or is there a better way using cross platform?
If you need to ask, you are going to need to hire someone to develop it for you. If you are considering this as a viable commercial concern, budget $100K-$500K and six months for the MVP to be on both platforms.