Choose a topic that you want to write your poem about. You do not want to start writing without any clue about what you want to write about.
Choose a rhyme scheme. Simple rhyme schemes are A A B B or A B A B. With A A B B, the first two lines rhyme and the second two lines rhyme and so on. For example, you might write: I saw the sky / Night passed me by / I saw the sun / I had some fun. With A B A B, the first and third lines rhyme, and the second and fourth lines of the poem rhyme. An example of A B A B is: I like the tall trees / So green and exciting / They sway in the breeze / Shade looks so inviting.
Choose a number of syllables for each line. Inconsistent syllables will not allow your poem to flow properly. For example, "The stars are bright / They shine at night" is more consistent than "The stars are bright / I sometimes wish it was light."
Make sure to have a strong opening and closing for your poem. You'll want your reader to become engaged in your writing and know when your poem is complete.
Feel free to stretch grammar rules a bit for the sake of your writing, but don't get too crazy with it. For example, saying "Watching the sun set over I" does not make sense when the sentence should be "Watching the sun set over me."
Have fun and be creative. A poem reflects you, so say what you feel.