I don't know if there's an online tutorial for Paint, but check the Help menu in Paint -- you might find some useful info there.
As for doing the mock-ups -- I just take a picture of the room that I want to do the mock-up of, then I look for pictures of things that I want to "put in" the room (like the mirrors and sconces in my dining room mock-up). I open my room picture in Paint, then I open the picture of what I want to put in the room in another Paint window. I select the item (sometimes I have to resize it), cut it, and then paste it into the room picture in the other Paint window. I use the dropper tool to select a colour to fill in the area around the picture that I pasted into the room picture, so it blends into the picture better.
I use the paint brush if I want to "paint" walls or items in the picture a different colour.
Hope that helps!! :)