I'm buying my GF a perfume or a real nice piece of jewelry, I think. It really depends on the status of your relationship. If it's a GF, get her a good gift, if it's someone you've been dating, flowers and candy are great. I would definitely get a girl some kind of gift on valentines if I took her out - not to impress her necessarily, but because I would just want to, as it is the masculine thing to do. Just make sure if it's only a date and not a GF that you do not go overboard, or else it comes off as needy or trying to buy her affection.
I wouldn't ask a girl out for Valentine's Day, personally, unless I dated her prior. Here's why: If the first date is on Valentine's Day, there is a chance, perhaps slim, that she is just going out with you so she HAS a date for that day - having a date for that day means a LOT to a girl. I would date her once or twice at least prior, then take her out on Valentines. If you have taken her out a few times beforehand, then getting her out for Valentine's shouldn't be a problem. If it is, don't date her again.