If you don't feel you can talk to someone you know, then there are a number of organisations, agencies and charities that can help for all situations. In an emergency you should always call the Police on 999.
CEOP is part of the UK's National Crime Agency, it is the best place to report online child abuse or grooming.
UK Safer Internet Centre
UK Safer Internet Centre promote the safe and responsible use of technology for young people.
Childline is a counselling service for anyone under 19 in the UK. They can help any issue they are going through.
Think U Know
Thinkuknow is the education programme from NCA-CEOP which protects children both online & offline
Internet Watch Foundation
The IWF help victims of child sexual abuse by identifying and removing online images & videos of their abuse
Internet Matters strive to empower parents and carers to keep children safe in the digital world.
NSPCC is a large charity fighting to stop child abuse on and offline. They have a helpline available for adults.