Hello! To start organizing, I would begin with your desk. (It's better to start small) Then I would maybe do your bathroom because, it's probably not super messy in there. Next I would do your closet and rest of your room. Hope this helped! If you need any more help, let me know. =)
First you have to decide WHERE you are going to put stuff. Get some cheap shelves like this: images.google.com/search?site=&tbm=isch&source=hp&biw=1024&bih=768&q=bricks+and+boards+bookshelves
Next get some cheap stackable boxes like this: www.homedepot.com/p/Sterilite-6-Qt-Storage-Box-in-White-Clear-Plastic-16428960/203120094 These are so useful that I got forty of them back in 1993 and I use them for everything.
Now that you are prepared, you can make a hobby of sorting things into boxes.