Need advice on what is the the best vitamins for hair, skin and just all the nutrients an 18 year old girl would need?

Food: If you're eating a really good, balanced diet, with lots of fresh fruits and vegetables (greens and other colors), lean meats and whole grains, then you're already getting most of the vitamins you need fo healthy hair, skin etc.. The one vitamin you should be taking is vitamin d. This is hard to get from your diet, and unless you live in the tropics you are likely deficient.

Calcium and folate (folic acid): Besides eating a healthy diet, young adult women should supplement with calcium and Folic Acid daily. Women with low iron might need iron supplements too, but you need to check with your doctor before starting that one because it needs to be individualized to suite your body.