Keep in mind that sex is not the same thing as romance so leave that out. Romance also doesn't necessarily mean flowers and candlelight. Rather, romance highlights the fact that you care about her deeply and feel a deep connection.
Write down everything you love about your girlfriend, from the way she smiles to the way she treats other people. At this point, don't worry about punctuation or spelling or form--just get the words on paper.
Look at what you have written and highlight or circle the parts you want her to know. These are the parts you are going to be weaving into your poem.
Group all the things you love about her into topics. For example, group together the physical attributes, then group together the way she deals with others, then make a group for her witty personality. These are the different stanzas in your poem.
Jot down your groups into a poem format. Remember that poems don't need to rhyme. If you want to put some structure to it, try counting syllables and create a pattern within your text. For example, have four syllables in the first line, six in the second line, four in the third line, six them forth line and so on.
Consider the length you want your poem to be and create your romantic poetry accordingly. There are many different styles of poetry out there, some poems fill a couple lines, and some fill an entire book, so don't worry about doing it wrong.
Vary your structure if you want. One way to do this is by writing her name vertically then using the letters on each line to describe her. Fox example Lin can be "Lovely, Inspirational and Necessary to have in your life."