Use a centered header of name, address, cell number. Update the cell phone mail box with a professional sounding greeting.
Add an email address. Use a professional sounding name; avoid unusual addresses like email@example.com. Set up the professional email to forward incoming emails to your main email so that you don't miss anything.
Under the main header of your resume, use a standard resume layout of which there are many free examples on the web. I DON'T recommend using a resume builder app because you will be forever beholden to the app and the format and mentors may have difficulty editing and reviewing the text. MS Word or an open source document format gives you the biggest flexibility.
Your first section should be ACADEMICS under which you list your most recent school/college first and work backwards to high school(s). Include a short list with bullets that showcases key classes where you did your best, activities like Key Club or photography, academic awards, and if strong, GPA and any standardized test scores of note.
The EMPLOYMENT section comes next. If you haven't had a job, try yard work; dog walking; pet or baby sitting; so that this section isn't empty. List the jobs chronologically, most recent job first, and include 3 bullets maximum per job with a focus on quantifying accomplishments (Note: in some cases, this section can be listed before ACADEMICS).
EXTRA-CURRICULAR ACTIVITIES go last. Make the bullets in this section accomplishment oriented. Bullets can be listed chronologically, or grouped under italicized sub-headers such as Sports, Theater, Music, Scouts etc.
Lastly, if there is room, add a one-line section under the header HOBBIES. Include activities like cooking, fantasy football, Magic the Gathering, fiction reading can go here and will often help make a meaningful connection with the resume recipient.
Parent and child should write this document together then review it several times over the course of two weeks to have the best chance of correcting typos and adding missing activities.