Use the first line as the title. It's the easy way out, but some poets do it.
You can tap the extended metaphor, which is usually more effective than using the first line as a title. For example, if the poem is an extended metaphor that describes loss, name it "Loss," or something close to that, such as "Lost," "Missing" or "Hidden." Add another word if you like, just as seventeenth-century English poet John Milton joined "paradise" to "lost," giving the world one of the most famous poems ever written, "Paradise Lost."
More effective still, locate a line in the poem that resonates symbolically and use it to devise a title. For example, perhaps you wrote a poem about someone's impending death. In it, you mention a stray black dog that will soon be put to sleep if no one claims it. You might name the poem, "Requiem for a Black Dog," even though the poem is essentially about a human being's death. Naming it for the dog lends depth and highlights the tragedy; it captures the senselessness of the death, and it suggests that in the end, we are all nothing more than anonymous creatures whose death will go unnoticed by the world.
Poets often name poems after the poem's reason for being. For example, William Butler Yeats gave poems names such as "On Being Asked for a War Poem," "A Prayer for My Daughter" and "Colonel Martin."
If you want, let someone else name it for you. The poet Donald Platt did this with a whole collection of poems. His wife named the collection after he had made six stabs at coming up with a good title. The worst that will happen is that you will reject the idea.
Cheat. Decide on an arbitrary formula by which you will name poems when nothing better comes to you. Some poets have been known to title a poem by doing something along the lines of choosing the third verb in the fourth line.