For your age I would go ahead and use your shotgun. Do not worry about a scope. I agree with buying a few different type of slugs and try them out to see which is the more accurate. At 13 this should be a good way to go as with it being only a single shot it will force you to take your time and think before shooting. Later on a scope and other items could be purchase when you get the money. Good luck and hope you continue to hunt and keep asking questions. Some forget what the OP asked and who he is.