Talk to one of your parents. I'm certain they're aware that you're grieving (feeling shame or worthless is a form of grief).
They can help you out, or even help you find other people who can also help.
You're at school right?
Your school/college should definitely have a Student Support department/officer, and they should most definitely provide bereavement support. They are invaluable, and are extremely supportive and helpful. Don't ever think that you're too old, or even too young to recieve or need such support.
This website won't provide information on any services available for you, for obvious reasons. But it does provide advice. Give it a read.
http://www.nhs.uk/Livewell/bereavement/Pages/bereavement.aspxAnd, I don't know if you are religious or not, but if you are, consider talking to someone from your religion. Whether it's your vicar, or anyone else who works in your church/temple.