Well first you need to know their contact information, which I'm pretty sure they've stated it clearly in that newspaper.
As for the application, tell them that you're interested and would like to apply for it. Unfortunately I can't be entirely specific about how you should word every sentence but, write out what comes to mind first. Here's some key things you can include:
- The fact that you're interested (obvious)
- Your past work/volunteer experience (if you're a high school student, I doubt you'll get marked down for having no experience)
- Some information about yourself (such as age, but I wouldn't recommend going into TOO much detail until you can talk to someone regarding the job face-to-face.)
- Mention YOUR contact information (i.e. Phone, email, address)