Personally I would start with the flowers, most of the girls feel very special when you get them flowers. If you want a jewelry, buy gold (some people are allergic to other metals, like myself, i can't wear silver). I white gold circle with diamonds is a nice gift or a white gold heart with the diamonds also is really nice.
Do not buy ring, there are different sizes rings,
Earing, pretty hard to find the one girls like, some girls prefer certain type of locks over the others, not sure about that, but don;t go for it.
Do not buy watches, it's a very bad gift.
IMHO Perfect gift for a girls birthday will be
bouquet of roses (they you buy roses make sure you take a look at them, if flowers are big and opened up widely, try to avoid those, they will dies in a couple of days) Also make sure there are ODD number of roses (Hard to explain why, you can find it online)
Necklace (already told you what to buy)
Victoria Secrets Gift card
Good Luck!!