Recommendations are very important in the admissions process. Just because someone is willing to write one for you doesn't mean it will be helpful. You should be asking people for whom you have displayed excellence in work habits, honesty and ability whenever possible. If you're not already a senior, you can target recommenders for especially excellent effort on your part. Always show your recommenders your best, and be sure to furnish them with your activity list well ahead of time. Never ask for a recommendation with fewer than a couple of weeks to go before it's due.
These articles will add detail:
Why Juniors should ask for recommendations 22
GETTING GREAT COLLEGE RECOMMENDATIONS18.DOCX
HOW TO ASK FOR A LETTER OF RECOMMENDATION FOR COLLEGE18.DOCX
THERE’S MORE YOU NEED TO KNOW ABOUT LETTERS OF RECOMMENDATION18.DOCX
WHO AND WHEN TO ASK FOR RECOMMENDATIONS18.DOCX