Apple

The very first thing you need to do before making an app for apple products is download Xcode. Xcode can be found on the app store or at the link below. However, Xcode can only be installed on computers with iOS, those made by apple. You will need Xcode on one of those computers in order to begin developing Apple apps.

https://developer.apple.com/download/

Apple itself puts out a massive compilation of resources for app developers on their website. This is really helpful as you gain more experience, but it is not the best place to go if you are just starting out.

https://developer.apple.com/swift/resources/

Apple also has a tutorial page here that goes step-by-step on how to get started making your very first app! This is great for those with little to no experience making apps.
Another good way to start learning (especially if you have no experience) is Code With Chris’s videos on Youtube. He makes great videos for learning how to make apps with Swift (the easiest to learn Xcode supported language).
There are, however, some constraints on publishing apple apps. In order to publish an app for the App Store you need a developer account with apple, and your app needs to meet apple’s app guidelines, found here:

https://developer.apple.com/app-store/review/guidelines/

The develop account costs money to maintain, so if you have a great idea or prototype you could send it to science helps and we might be able to publish it for you.

Android

The very first thing you need to do before making an Android app is install Android Studio.

https://developer.android.com/studio/index.html

Android also puts out a wealth of resources for prospective app developer. This index includes a massive variety of resources but is not the best place to start if you are coming in with 0 experience.

https://developer.android.com/index.html

Additionally, Android has a step-by-step tutorial page on creating your first project for those just starting out. This page is perfect for anyone with no experience making apps.

https://developer.android.com/training/basics/firstapp/creating-project.html

Another good way to start learning if you have very little experience with making Android apps is the videos by thenewboston.

https://www.youtube.com/watch?v=QAbQgLGKd3Y