cajunmama

I am also a beginner to woodworking. My questions are also very basic one. How do you know what size dowels you would need to fill the holes? Also, do you keep dowels of various size and species around the shop or do you make them as needed? I have to order my through the mail. What sizes would you suggest I order? I know this is a broad question but I have to stock things in advance before I start making a new project.