Sometimes flowers in a rolled up newspaper are the most romantic way to go. Vases are generally pretty cheap at florist shops though, so if you have the extra change, just pick one up also. Online ordering shows foresight and planning, but the corner store can show what a romantic sucker you are. Look for seasonal specials. If you have read this far, you must be a novice.
Some flowers have meanings, check out some common ones here.
Don't stress out too much about picking the perfect ones. Most will just love that you made the effort. While I come across a little glib, women can smell a scam a mile away. And for god's sake, send them to where she works, make her the envy of her co-workers.