I am a beginner in this tool what should I do?

The first step is to learn the basics of PowerApps, Microsoft offers free and very useful training, I share the links with you to:

Introduction to Power Platform

https://docs.microsoft.com/en-us/learn/modules/introduction-power-platform/

Get started with Power Apps

https://docs.microsoft.com/en-us/learn/modules/get-started-with-powerapps/

Create a canvas app in Power Apps

https://docs.microsoft.com/en-us/learn/paths/create-powerapps/

Customize a canvas app in Power Apps

https://docs.microsoft.com/en-us/learn/modules/customize-apps-in-powerapps/