I am a very sentimental person and have a hard time letting go of things from my childhood. Because of this, my closets are full of old books, toys, CDs etc. I'm running out of space to store important things such as clothes. I have this same problem with my desk. Does anyone have any cleaning out and organization tips for me

I have a hard time, too. I make three piles: donate, save, and maybe. I immediately donate what I don't need. Then, I put my maybe pile in a spot in my garage. If 3 months goes by and I haven't needed it, I donate it. I try to clean out my closet every few months and I either keep, donate, or "maybe" the clothing. Every week when putting laundry away, I clean out one dresser drawer. If it's not something special from your childhood, consider donating or selling it.

I go through what feels like hormonal swings from extreme hoarder to extreme cleaner. (My mother was such a hoarder that her house was a health and fire hazard.)
When I get the urge to purge my belongings, I do so as fast as I can before I regret any of it.
I have a 3 month rule too, although I don't write it down. Just approximate.
My town dump has a 'swap shop' and I enjoy taking loads of stuff there, and watching people grab some of it, some of which I found there.

But the BEST fun is having a tag sale, and it's that time of year! Last one I made close to $400.
I also sell on ebay, and send things to family far and wide.