When your mouse clicks the canvas, print it's coordinates to the console. You need to print the coordinates of the mouse relative to the canvas, so make sure that the top left corner of the Canvas registers as at (0,0). If it isn't, figure out how to adjust your output so that it is.
Now when your mouse clicks the canvas, have a dot appear. Make dots continue to appear, following the mouse, until mouseup.
Add buttons that change the color of the paint brush.
Add a clear button that clears the Canvas when clicked.
Make two more buttons, a paint can and a brush. If the paint can is clicked, instead of drawing a line have the whole canvas turn the selected color. Then, when the brush is clicked have it go back to drawing.
Add an input option that lets the user control the size of the paint brush.
CHALLENGE: If you paint quickly, you'll notice that the line gets dotty. Can you figure out how to make your paint brush paint connected lines instead of dots?
Keep working on your Paint Application until you're satisfied with it. You can add cool features (like a rainbow pen), allow the user to input their color choice using a hex code, add a stamp feature that stamps cool pictures, etc!