How do I ask for recommendation letters?

i'm a grade 12 student and will soon be applying to universities

i need some rec letters from teachers but i'm not quite sure how they work

are the letters just written by my teachers and sent to the administrators to the universities?

or is there a certain format of rules by which my teachers need to follow when writing that letter?

and how should i go about asking these to my teachers?

i'm not that impressive of a student so i'm quite embarassed