Think of Canvas as a paper. When you draw something on a paper you are drawing a path of the character. Similarly on canvas there is a path. You can let user draw their stuff and record their path.
On paper you use different colored pen to draw, in canvas that functionality is provided by Paint object.
So you need to learn following
Custom View : How to create your view in which user will draw their stuff.
TouchHandling : You can set your view to receive touch events. Think of device screen as 2D graph. Whenever user moves their finger on screen your view will get touch event with x,y co-ordinates of current point. You can use Path to join together points to display user’s drawing on your canvas.
Paint : provides styling for drawn stuff.
Finally to save notes generated by user, you need to save your path and associated styling info in DB. For this, preferred approach would be to save(draw) user stuff onto a bitmap and save that bitmap in DB.