Filling out a birthday card for a girlfriend is a personal experience that depends on your relationship and type of girl you are seeing. Some girls might love a funny, off-colour card, while might prefer a sentimental and heartfelt expression of romance. What you put on the card is up to you, but there are different approaches to take.
Keep it Simple
Sign your name and write "Happy Birthday." It might be better to stick with something simple and effective than take a chance on a message that could be misinterpreted. Playing it safe may not earn you any points, but it also is less likely to blow up in your face.
Make it Funny
Try a funny message if your girlfriend has a wicked sense of humour. Make up a poem that starts with "Roses are red, violets are blue..." or express your feelings through a haiku. Reference an inside joke between the two of you to make the card funny and personal.
Make it Sweet
You can't miss with a sweet, honest expression of how much you care. Writing "I love you" or "You mean the world to me" might sound cheesy, but your girlfriend may appreciate an outpouring of emotion. Try quoting her favourite song or referencing a romantic poem for a inspiration if you are having a difficult time thinking of something sweet to say. Cite your work so she can listen to it or look it up later.
Make it Dirty
If you are dating a woman who does not mind walking on the wild side, punch up her birthday card with something a little scandalous. It doesn't have to be overt and offensive. Something as suggestive as "You're getting your real present later tonight" does the job. Find a way to be slightly dirty in your message without being vulgar and it might pay off for you as well.