At some point, you will need to ask for a letter of reference from one of your professors. They expect to be asked; it’s also part of their job. But, remember that who writes and for what matters. Don’t ask your prof who only taught you one intro course to write a letter of recommendation for graduate school. Why aren’t you asking your advisor who supervised you on an upper-level research project?
Also, make sure you provide adequate advance notice (minimum of a week)!
Three points addressed in this article:
Who to choose and when to approach them
What to say and what to give them
Thank you etiquette