What's wrong with me? am i depressed?

I'm only 15 and for a while now i just feel as if i can't be bothered doing anything i just want to lie in my bed all day. I wake up in the morning and just lie there until just before my mum gets in from work because i don't want her to think somethings up so i sit and put the t.v on but i don't actual watch it i just sit there, i don't feel up to going out and i always make myself feel worse because i think about things that make me sad like Heath Ledger's death (was a huge fan!), what's wrong with me? and how can i get back to normal?