A topic. What are you going to write about? Since haiku is such a short form, you may consider staying away from the more abstract concepts. At least until you have more experience. Consider writing about a topic that most people will recognize.
Now comes the hard part. The writing of your poem. In Japanese, the convention of having 5,7,5 syllables in each line is a hard and fast rule. But, since English can have different syllable length, this can be more difficult to accomplish.
Cutting your haiku (no, not literally). Consider using the cutting technique in your haiku to give the two portions of your poem a limited form of independence. To make this cutting in English, either the first or the second line ends normally with a colon, long dash or ellipsis.