I want to make an application (to draw and calculate the area of a circle)

Hello, i want to make an application that will accept from the user the radius of a circle in centimeters and will calculate and display the area of ​​the circle . Can you help me? step by step

You need a textbox to insert the value (if you want with some control about only number, decimal values etc) a button to calculate and a label to show the result.
When button click set the label text with the math expression you need