Choose someone who knows you well. If a scholarship is based on community service, ask someone who works with a non-profit you served. If it is based on academics, ask a teacher who know your character, your dedication, your academic achievements.
Remember, lots of students are asking for letters of recommendation at the same time. Ask early. Make sure you tell the person you are asking what the deadline is. Ask if they can submit it before the deadline and check back to make sure they submitted it.