I want to take screenshots by hitting one key on my mac os keyboard, but how?

Try and achieve this by creating a shortcut:

Create a new shortcut

1. In the Shortcuts app  on your Mac, click  in the toolbar.
2. An empty, untitled shortcut appears in the shortcut editor.
3. 
   
4. Click Shortcut Name, then type a name for your shortcut.
5. Select a category from the sidebar on the right (Documents, for example), then double-click an action in the list that appears.
6. You can also drag an action from the list to the position you want in the shortcut editor.
7. To add another action, repeat step 3.
8. When you’ve finished adding actions to your workflow, click the close button.
9. The shortcut appears in the main Shortcuts window, in the All Shortcuts category.
10. For information on customising the appearance of your new shortcut (the rectangle in your shortcuts collection), see Modify shortcut colours and icons on Mac.
11. To test your new shortcut click .
12. To stop the shortcut, click .

Tip: You can also create a new shortcut by Control-clicking the Shortcuts app in the Dock and then choosing New Shortcut.

For information on rearranging your shortcuts collection, see Reorder shortcuts on Mac.