Before you start writing your love poem, you should think about whether you want to compose it in a traditional, structured verse form or write lines in free verse. Structured verses are traditional poetic forms with strict rules controlling factors like a poem's rhythm, the location of rhyming words and the number of syllables. Writing in structured forms is helpful if you find that guidelines help stimulate your creativity, and if you want your poem to have a touch of the classic. Some common poetic forms include the sonnet (associated with poems by Shakespeare and generally considered quite romantic), the limerick (associated with humorous and bawdy subjects) and the haiku (a short Japanese form without rhyme generally used for descriptive passages associated with nature and seasons). For more forms and instructions about how to write them, check out the links in the "Resources" section below.
With free verse, every decision about the structure of your words is up to you. Free verse poems generally avoid any attempt at musical rhythmic and poetic structures.
When trying to decide what sorts of sentiments you want to express with your poem, it helps to decide on a subject. Descriptions of feelings are difficult to express artistically in a poem, but describing a physical subject or a moment in time as a reflection of those feelings is a common way to distinguish poetry language from everyday speech or prose.
For example, if you're trying to write a poem about how beautiful your lover is, the subject of the poem may be her physical features, or a single feature like her eyes. However, you may also do something more abstract and surprising that describes the effect of her appearance rather than details about it, like personifying a mirror and speculating about its reaction to seeing a woman like no other, or discussing the journey of starlight across light years of space to grant light to see one's lover.
Commonly, poets of renown will describe the art of writing poetry as the search for not just the right words, but the "only" words. This is subjective, of course, but most poets agree that the best poems are the ones that go through a lengthy course of rewriting and refining.
With a love poem, try to express sentiment that is both heartfelt and personal to you. A good way to start getting in touch with these sorts of feelings and words you can use for them is to do some stream-of-consciousness writing (an exercise where you start writing and don't stop, putting down whatever words come to mind), then look for good turns of phrase to help you start your poem.
Ultimately, writing a love poem for someone you care about will probably be more about a personal expression between the two of you than any concerns about a major artistic endeavor, but you might still consider some guidelines of good poetry writing. Avoid too many adjectives and try to use fewer words where you can. Read your poem aloud to yourself a few times and pay attention to the rhythmic flow of your words (whether or not you're writing in structured verse).