How to know what to delete ??

SO I need to clear some space in my mac, a lot of room is being taken up by "Other." I have DiskUtility X, I was just wondering what files are safe to delete and what are not, aside from Apps, pictures and such. I mean like the complicated sounding things (For example, i have .sidd and) . Sorry if I'm explaining this poorly; there's a lot of folders that don't really have good names (ie; they were pre-programmed into the mac.) I guess what I'm asking is; what are ALL the types of files okay to delete? I don't want to delete something and then find out I actually needed it for the computer to operate.

Specifics (file types, .___) would be nice!

Thank you!