Writing in first person means writing from the author's point of view or perspective. This point of view is used for autobiographical writing as well as narrative. The first person is an alternative to second person, which uses "you," as in the sentence "You are the smartest person in the room." The first person is also an alternative to third person, which uses "he," "she," or "it," as in the sentence "He is the most handsome person in the room."
Are you nobody, too?
Then there's a pair of us -- don't tell!
They'd banish -- you know!" - I'm Nobody! Who are You? by Emily Dickinson
Over many a quaint and curious volume of forgotten lore" The Raven by Edgar Allan Poe
When all at once I saw a crowd, A host, of golden daffodils;" I Wandered Lonely As A Cloud by William Wordsworth
A blameless wight,
Who, for a year or more,
Had daily passed my door,
Yet converse none had had with him." I Knew a Man By Sight by Henry David Thoreau
When I say that something, I wanna hold your hand" - Beatles
My father was a gamblin' man down in New Orleans" House of The Rising Sun - Animals
On a blanket with my baby, Is where I'll be" Under The Boardwalk - Drifters
Because you're mine, I walk the line" I Walk the Line - Johnny Cash
I know that you have 'cause there's magic in my eyes" I Can See For Miles - The Who
And the way the sunlight plays upon her hair" Good Vibrations - The Beach Boys
These examples help to show how to write in first person.