If the prompt is simply to write about the subject of friendship, you have a couple of options for how to go about writing. In my classroom, students would read a prompt like this and know to write either an extended definition essay or a compare-contrast essay. In this case, extended definition might be easier.
In order to begin this paper, you must ask, "What does friendship mean to me?" Follow this question with another, "Considering the idea of friendship, what have I experienced personally, observed, or read about, that would help me explain what it is?" Then, make a long list of your experiences, observations, and things you've read that help you to explain friendship. It is very important in this step (as well as in this essay) to explain friendship through examples. Simply speaking from a philosophical (and therefore detached) point-of-view will be too generic.
Once you've brainstormed ideas in each of the above categories, consider a way to define friendship in one or two sentences. This will be your thesis statement. One way this essay could become a compare-contrast essay (in addition to definition) is by saying what friendship isn't as compared to what it is. Once you've put down a one or two sentence definition (thesis) your three subtopics (and therefore, three body paragraphs) easily become 1) experiences, 2) observations, and 3) readings.
Remember to use plenty of real examples. Elaborate on and explain your examples as they apply to your thesis statement.