how do i delete memory of my acer?

my memory is full on my acer aspire 3689 and i dont use anthing that is on here. and the things i want and need to download to my laptop wont because it says i dont have the room for it. how do i delete the things i dont use and dont need with out messing up my laptop.i dont know what half the things on here are and if hey help my computer run or what. please help me coz i really need this computer but im new at all this stuff. thank you.