You can ask anyone with whom you've worked to write you a letter of recommendation. You'll want to choose someone who knows you and how you work, rather than someone who has an "important" name, someone who can describe you in a way your CV and application won't. The goal is to get them to use an example of how you can go above and beyond in the letter.
When you ask your writer be sure to provide a CV and personal statement, and don't forget to remind them of a scenario in which they taught you something, and you excelled. Also let them know how to upload the letter by attaching the link to Letter of Recommendation instructions.
You don't! The letter writer should upload the letter of recommendation themselves by following the instructions provided by AAMC. In the past they were able to send the letters to our office, but that is no longer allowed.