Hi Im almost 13 and I think what you can start by doing is stretching yourself out (ing as you use quite small objects).If you only use one finger slowly start trying two. It might hurt a bit but you will slowly get used to it. After you're able to fit two fingers, use only two fingers for about a week (or less). By the way you can try curling your fingers into a "C" and using the "Come here" motion with your fingers. Then when you're fully comfortable with the feeling, you can start using other things. If wanna use vegetables then I would start by using the handle to your hair brush.. And as always don't rush it when putting anything inside your self, take it slow. Remember that when beginning to masturbate make sure your
properly wet enough. To help with that you can read smut on Tumblr, (If you have tumblr) watch porn, or anything that can make you Wet.
put a wash towel on the pillow and hump, it feels amazing (make sure wash towel is dry)