How do you organize a teenager's room with limited space?

I'm 18 and graduating highschool this year, and before I start working and going to university, I would really like to organize my bedroom. The big issue is that before it was my bedroom, it was our playroom/storage area, so over 80% of all of the shelves and cuboards are filled with everything from our old baby toys to my mom's art to my dad's model rocket supplys, I mean, I have the ALL the winter coats and mitts and stuff in my closet! I don't want to complain, but my parents keep saying my room's a mess, but where can I put all of my stuff? the storage places under our back deck and satirs are really cluttered, so I can't just move stuff there eather, any suggestions?