Depending on whether you are in a serious relationship or just dating and banging occasionally.
Serious: flowers, dinner and jewelry! Doesn't have to be super expensive but a necklace or bracelet or pair of earrings are very special to a girl, small and meaningful is best.
Not serious: flowers and dinner.
Most nice restaurants do a special menu on Valentine's Day that is way over priced so if you can cook at all, make her dinner at your house. Or, if you can't cook, take her to her favorite restaurant if you know what it is.
Most importantly, be a gentleman and let her finish first