I'm glad to see young people like yourself getting involved and giving back.  As for selling you plan to your principal, here is a simple business plan that you can following.

1.  Decide what you really want to do.  You mentioned to different ideas: service club and magazine.  I would pick one or the other to get started.

2.  Get other students interested.  Make sure you have a commitment from others that they will help you.

3.  Try to line up a mentor.  This can be teacher or community member.  For example if you wanted to start the school magazine, see if there is a local report who will help you out and give you advice.  If the person can't be there all the time, maybe they can be a virtual mentor giving you advice via email or phone calls.

4.  Find a teacher sponsor.  Most schools in the US, clubs or service organizations have a teacher contact.  This may allow you to use that teachers classrooms for meetings and they can give you advice as to what would be acceptable in different situations.

5.  Develop your plan.  It should include three parts.
The Problem - what problem are you trying to solve.  Let's stick with the magazine idea - the problem might be that teachers and students don't know what is going on in the school.  They don't know when sporting events take place, when clubs are meeting, or what's on the school lunch menu.  
The Action - Because students and teachers don't know all the cool things going on in the school, a school magazine could solve this problem by keeping everyone informed.  It is also at the time that you let your principal know that you have a mentor and teacher sponsor ready to help you.  Describe responsibilities and those of the other people involved.
The Result - what result are you looking for?  Do you want to list events and date?  Do you want to highlight teachers, subjects, projects in your school.

6.  Ask to set up an appointment with your principal.  On that date, dress like you mean business - look nice!  Give your idea pitch to your principal in a positive manner.  Point out that your idea will make the school better.  

Well, this will get you started.  Be sure to read what others offer as advice.  I'm sure there ideas will all spark ideas for you.

Good luck!!