ooh i can help you out a little with sai & digital art in general if that's okay? c:
drawing tablets
generally, lining your art and colouring it in is far less time consuming with a drawing tablet, so if you end up getting a bit more interested in digital art i'd consider investing in one of those; wacom bamboo ones are great and not too expensive compared to what you might be forced to pay for others. (for reference, i use a wacom bamboo ctl-470, which is pretty basic but still works very well.
however, i understand they can cost a fortune, so you probably won't feel the need to get one for now, but i thought i'd say a little on the topic just in case.
about layers
messing around with layers in sai is actually pretty simple. below, i labelled the top 3 functions you'll want to use:
as you can see, with those 3 buttons, you can create new layers, merge a layer with the one below, and remove layers completely. to move the arrangement of layers (i.e. whether layer x is above or below layer y), you just drag and drop them within the mini box beneath the buttons.
i hope this helped a little bit!