I have an Acer desktop with all the memory used up on C drive?

How do I resolve this? It's a household pc...I have most photos & music on a separate hard drive and a variety of programs but nothing more than my friends who have lots of memory left. The fact that hardly anything is left on mine (it shows in the red on C drive) has me really confused and I don't know what to do next to try to clear space....any suggestions?