When in doubt, go and see a professional! These sites are intended as a guide and should not replace advice from a medical professional.
Please remember you can also ring 0845 46 47 to speak to NHS Direct, who can advise you. (UK)
First Aid advice from the St John Ambulance. Covers cuts, burns and poisons. They also hold first aid courses throughout the country.
Everyday First Aid from the Red Cross
First Aid advice from the NHS website
How to treat Burns from the NHS website. Covers when you need to go to hospital.
How to Treat Cuts and when to see your GP/hospital – NHS
Self Harm: Limiting the Damage from the Royal College of Psychiatrists. This pdf contains information on treating common injuries, how to stock your first aid box, and more.
Self Harm First Aid from Recover Your Life
First Aid for Self Injury from Secret Shame
First Aid for Self Injury and Self Harm from LifeSIGNS