If you have never written a poem before, you may just like to write her or him a nice letter instead. It's easier to express yourself and just as romantic.

But if you do want to try writing poetry, the best advice is probably to keep it simple. You don't have to rhyme (in fact it's better not if you haven't had much practice) and it's fine to write 'free verse' with lines of different lengths. Why not start by writing down all the things about your sweetheart that you really love, or that make them special. Try to mention these in a straightforward way - imagine that a third person has asked you 'why do you love X?' and that you are trying to explain it to someone who's never seen him/her before.

You can also look at some other Valentine's Day poems for inspiration. This site also gives you some tips for writing your own message.