Calling All Vitamin Gurus!!!

Okay, I've decided I need to step up my health game... a lot. One thing I want to start doing is regularly taking vitamins but I honestly have no idea what I need or what vitamins do what.

About me...

•Female, 20 years old

•Mild acne (I get the occasional 'under the skin' pimple)

•Semi-active lifestyle: I walk my dog daily and horseback ride 2x a week

•Tend to either sleep for 3 hours, or 14 hours- no middle. Also wake up a lot in the night.

•Very brittle nails from years of nail biting- Id love to make these bad boys stronger so I can kick that habit

•Tend to experience seasonal depression

•Poor diet- because I'm blessed with an extremely high metabolism and have always been under weight, I usually eat a lot of carbs and take out. Pizza, pasta, etc.

I'm thinking probably Vitamin C and D... but are there any others? I've tried "Hair Skin and Nails" gummies but don't fine they help much- any other suggestions for that?