Hi Mateo,
Make sure you read ALL the information concerning Jr. L1 certification on the NAR.org website.
https://nar.org/content.aspx?page_id=22&club_id=114127&module_id=668315
The information on the website will answer your questions.
This link will point you to information concerning the Jr. L1 exam.
https://nar.org/content.aspx?page_id=3030&club_id=114127
Here are some guidelines that CMASS is developing concerning HP certification. The webpage has not been officially released yet but there is a lot of useful information. https://cmass.org/L1CertAtCMASS
HP motors are not available at the field. You must be 18 years old to even touch a HP motor.
What experience do you have building and flying mid power rockets?
Why do you want to be Jr. L1 certified?
Have you used software such as Open Rocket to run simulations for your rocket?
Frank DeAngelo