Freeing up storage space on Hard drive and backup

I am a dropbox user and recently found out that the Sync function in Dropbox loaded about 250 GB onto my hard drive and that also resulted in a big overload in my backup drive ( I use the Iomega 1 TB MiniMax). Does anyone have any suggestions as to how to delete some of this dead storage and free up some space. I use the Mac Mini with 2.3 Ghz and OS X.9.4

Select, Double Click, Move to Trash