You have the typical symptoms of a fungal infection called Thrush.
You are in the UK, so make an appointment to see your doctor who may get a nurse to take a swab to be sent to the lab to identify the bacteria/fungus or by your listening to your symptoms prescribe you with either a cream like Canesten, and/or a one dose oral capsule called Fluconazole. You can buy these without a prescription at the chemist. The Fluconazole costs £10, don't know the price of the Canesten.
Do not use soap or shower gels or bubble baths as this aggravates the condition and makes it worse. Only wear cotton pants, don't wear tight trousers or tights. Fungus loves warm and humid places and a sweet environment. If you have a diet that consists of a lot of very refined and surgary foods and fizzy drinks, you should cut them out or drastically cut down.
Use non biological washing detergent to wash your undergarments as sometimes an irritation can occur from the biological detergents. Use fabric softeners that are suitable for sensitive skin.
If you are overweight, that can help by getting rid of folds of skin where the fungus can flourish.
Use of antibiotics can bring about the condition of a fungal infection. So if you are prescribed with an antibiotic do ask your doctor to give you a prescription to treat fungal problems after you finish the course of antibiotics.
People with diabetes need to control their sugar levels to help prevent Thrush.
Don't suffer the misery of your condition and make an appointment. Although itching can lead to scratching, avoid scratching because it can damage the skin and cause a condition called Lichen Planus.
You can use non perfumed soaps or the Simple soap and those with a pH level that is kind to our skins.
You can have a bath with a handful or 2 of oats. The doctor may also prescribe an antimicrobial bath emollient like Dermol 600.
As a home remedy, wash and gently dry your genitals and apply natural set yogurt. This will help to cool down the itching and soothe the soreness. Yogurt also helps to lessen any inflammation that is likely to be present. It does work.
If it's itchy it might be an infection. You should probably see a doctor. I had a yeast infection once with those same symptoms. I went to the doctor, they took a swab, and gave me an antibiotic. No big deal. It's really common for girls, I'm sure if you told your mom, or an older female you trust they would understand and help you. No need to be embarrased. :)