Applying to college and graduate school are two of the times that most people will need references. It can be hard to know whom to ask when you're looking for someone to speak to your abilities and talents. Basically, you have to find the balance between three major factors when considering a candidate to write you a letter:
1) How relevant their expertise is to your pursuits
2) How well they know you
3) How much professional clout they have
When you're seeking a position in college or grad school, having someone who really knows the field you're going into is a major plus. If you're applying for sports scholarships in college, having a recommendation letter from your team coach is a great idea, because they can speak to your teamwork, school/practice balance, and talent on the field. If you're going into a graduate program for creative writing, you'll want a teacher from the English or Creative Writing departments.
For academic references, you'll need a school faculty member or teacher. In college, the hierarchy can be tough to navigate. A GTA or adjunct professor can seem like the best bet because they actually know your name and can speak to the quality of your work. However, they're also at the beginning of their careers. Many of them are also in school, lack a PhD in their field, or haven't established their credentials yet. Most of them are part-time teachers, and they won't get paid for the time they spend outside of their classroom writing your letter, which also makes them unlikely to accept many recommendation requests.
Another bad idea is going for someone who is highly credentialed but who knows nothing about you. Lecturers and the dean of the school might look really good with your application, but if they don't know you, they won't be able to sell your strengths. And a tepid, uncertain, or noncommittal reference letter is worse than none at all.
The best thing you can do is think about this in advance. Take a lot of classes in the degree you want to pursue and make sure you speak up in class. Go to the professor's office hours. Choose a professor whose middle or advance class sizes are small, but who still is a lecturer for entry-level classes. Those teachers have the credentials, they have relevant experience, and they'll know you fairly well. Just make sure you ask fast, since other students will be thinking the same way.
Index of letter of recommendation templates