You put your own reputation on the line when you write a letter of recommendation. If you feel you do not know the person well enough, or they are somebody you cannot in good conscience recommend, decline their request.
You are required to have three letters of recommendation submitted on your behalf. The letters should be submitted by instructors in a discipline related to your proposed field of study who are in a position to analyze your abilities and academic promise. Letters may be submitted online or on paper via regular mail; online submissions are strongly preferred.