When we think about a love poem, we often think of a poem describing a person's romantic feelings toward another. Thus, we get Elizabeth Barrett Browning's "How do I love thee, let me count the ways," and Robert Burns's "My Luve is like a red red rose / That's newly sprung in June."
But there are more facets to love than just the romantic side. The fact that you haven't yet had your "first love" doesn't mean that you don't know anything about love. There are people in your life whom you love. Think of your friends, your family, or others who have shaped your life in some way. Write a love poem to a grandparent, to a sibling, or to your mother, as Edgar Allan Poe did in his "Sonnet - To My Mother":
Because I feel that, in the Heavens above,
The angels, whispering to one another,
Can find, among their burning terms of love,
None so devotional as that of "Mother," . . .
You could also approach love more generally. What is love? What does it mean to love? Consider this final stanza from Sylvia Plath's "Love Is a Parallax":
Then jet the blue tent topple, stars rain down,
and god or void appall us till we drown
in our own tears: today we start
to pay the piper with each breath, yet love
knows not of death nor calculus above
the simple sum of heart plus heart.
Here, Plath states that love is bound by neither death nor science (as represented by calculus), but also shows how simple love really is.
Because the best love poems are intensely personal, I think the best focus for your love poem is contained in your question. "I'm only 13 and I haven't had my first love yet." Believe it or not, that's a great starting point for a love poem.
You've certainly heard about love, seen it portrayed in movies and on TV, and read about it in stories and poems. Based on your secondhand experiences, what do you think love will really be like, or what do you hope it will be like? Does the thought of falling in love scare you or thrill you? How do you think you'll know when you're in love? How will it change you, or how do you hope it will change you?
A poetic answer to any of these questions will make a wonderful love poem that you'll want to reread again and again, as you become older and more experienced, as a reminder of your youth, your folly, and your idealism.