Your daughter's 18th birthday is one of the most significant milestones in her life (and in the lives of her parents), so you'll want to include a message in her birthday card that's suitably memorable. Don't allow yourself to be too daunted by the blank white space facing you -- there are various ways in which you can approach it, and if you find that one angle isn't working you can switch to another.
Make it personal
Make your message something that's personal and unique to you and your daugher -- avoid including a standard greeting or poem that could apply to any daughter and any parent. You could include stories, anecdotes or jokes that you've shared over the last 18 years, for example. No one knows your daughter better than you do, and that should come through in the message you write.
Make it memorable
Your daughter only turns 18 once in her life, so make sure your message is memorable and includes sentiments that stay with her. Aim for profundity rather than flippancy, and (if you can remember that far back) try to recall your own feelings on turning 18. The more time and thought you put into the message, the better your chances of coming up with something memorable.
Your child is to all intents and purposes an adult when she turns 18, so take the opportunity to offer up some advice for the future. Avoid being heavy handed (you might want to include one light-hearted suggestion and one more serious one, perhaps) but don't be afraid to pass on some wisdom that you think your daughter might need in the years ahead.
Your daughter may be a supremely confident young woman or a terribly shy recluse, but whatever her personality, you can encourage her through a birthday card message. Pick out some aspects of her character that you like the most, and compliment her. Parents are expected to dote on their children, so the more unique and thoughtful the praise, the more it will stick in her mind.
A well-chosen quotation from a poet, writer, TV personality or celebrity might go down well -- avoid choosing overused phrases and pick something specific to your daughter (from a book she's read or a film she loves for example). The right quote could make her laugh, give her advice for the future or remind her of a happy memory.