how to delete files from icloud

i have been recently getting a message whenever i charge my iphone that my icloud is full and my files are not backingup, so i wanted to know how the **** can i delete some of my files which are already on the icloud because i am pretty sure a lot of data on the icloud is just garbage so i want to delete it. so that i can get some more storage space. i am gettting really anoying from these messages and also i have downloaded the icloud app on my windows 8 but still have no luck i cant find any pictures or songs or any contacts i really need to know how to delete that data because a lot of my files do not backup because i do not have space......

THANKS!!!!!!!!