By default on iOS or iPadOS there will be two different ways of taking a screenshot, depending on the version of the iPad or iPhone.
With a Home Button
Simply press the lock button (top of the iPad) and the home button at the same time to take a picture of whatever is currently on the screen.
Without a Home Button
Simply press the lock button (top of the iPad) and the volume up button at the same time to take a picture of whatever is currently on the screen.
After one of these options has been used:
A small image will pop up as down the bottom right that you can click on and it will bring up an annotation screen where you can crop and draw on the image. if you click on it, or you can swipe it away and it will save directly to the Photos app.