Just because a instructor gave you an A doesn't imply you have to ask him or her for any college letter of recommendation. Actually you may want to ask someone who noticed you struggle a bit during class, and then overcome that barrier. Feel free to discuss your options along with your college counselor, or straight ask the teacher which you have chosen if he or she feels comfortable writing the recommendation for you. It's your work to make sure the teacher is actually excited to write to a school on your behalf. You won't actually reach see these letters, so you want to be sure you have a good feeling so that they're going to say.
Remember: if you're basically assigning your educators homework. In addition to giving them lots of notice, you must also deliver them all the materials they have to write a great letter. You may want to fill out a form from the college or university, provide credentials and information to get a college's online application program, or get an addressed and also stamped envelope.