Best option: don't start from scratch, start from a template/sample (File menu)
Next best option: look in the tutorial section and read lots.
Otherwise try this:
Choose menu File/New
Choose "New empty project" (Enter)
Double click anywhere
Double click "Sprite"
Click the white area inside the dashed lines
Click inside the square in the window that appears (it should fill with a colour)
Close the Edit image window
Click Behaviors link under Properties (left of screen)
Double click "Bullet"
Close Sprite:Behaviors window
Click on the white area
Click Project Properties "View" link under Properties
Next to Preview browser, choose "Node-Webkit"
Click "Play" (Run) button in tool bar
You should see a window open and a green square move across the screen.
Congratulations, your first 'game'.