Do you have any other symptoms along with the itchiness? It may be a sign of infection and that should be seen by a doctor. Where exactly is the itchiness? If it's on the outside of your vagina, labia lips, it may be from ingrown hairs or from irritation from sweat/discharge. If it's on the outside, just try to keep the area clean and dry. Don't wear too tight of clothing and wash the area with just plain soap and water to keep it clean.
It can actually dry out the sensitive mucus membranes of this area and should not be applied. One thing you can do for now is to very GENTLY use a cool wash cloth. Do not apply any pressure with the wash cloth. Pat dry with the towel, but do not rub. Then, be sure to wear LOOSE fitting clothing and be sure to wear all cotton underwear. The area needs to be able to breathe. No vanity clothing, because that is usually rather tight fitting and will aggravate the problem and make you more uncomfortable. Try these things first before doing anything more. If the problem doesn't clear up, you would then see a doctor who can prescribe the correct type of cream, if you even need anything. Sometimes girls get a slight bacterial infection rather than a yeast infection. Often, it can be prevented with the above suggestions. Definitely don't use the Benadryl, because it will dry out the sensitive mucus membranes. It is also the wrong medicine. Benadryl is an antihistamine type of anti-itch medicine, but it won't help with female type of itching. You don't want to dry out the mucus membranes in that fashion, because it will cause you more problems.
You might have a yeast infection, so you should look up info on that and check to see if you have any other symptoms.