Most institutions will require you to submit at least one but up to three letters of recommendation as part of your application package. These should generally be written by two of your teachers. Your referees should be recommenders, and familiar with your activities in and outside of the classroom, so an alternative third referee might be a coach from an extra-curricular activity, for example.
You should meet with your recommenders, and discuss what you would like them to highlight in their letters. They should present you in your best light and avoid being modest. Recommenders should submit their letters directly – they will be uploaded as part of the Common Application or other application system that the university is using. You need to make sure your recommenders are fully aware of any specific submission requirements and deadlines.