It is a very good habbit to keep everything separated in layers in photoshop. even the most minor thing that will save you a lot of head ache. so New layer filled with white on top of the scanned layer
Drawings. As you can see this is a major improvement from the scanned drawing. This was done using the path tool, Usually I will use a brush of 3 Pixels anywhere 80% - 100% opacity. Important thing to remember is all the line tools of illustrations go on top of everything. i made a folder and inside that folder I have all the lines. From now on all the new layers will be below that folder. Path tool needs some practice with the mouse until your hand gets steady and you can get clear paths.