In a school setting where a lot of people will already know of you or where it will mean something that they don't, your reputation can play a big role in getting elected to a position like school secretary, but a great campaign with powerful posters and a thoughtful speech can make anyone win.
Learn About the Position
Before you can create a speech and posters that convince other people you can do the job of school secretary well, you need to make sure that you are prepared to do the job, and even before that you need to make sure you fully understand what the role of the secretary is. If you haven't previously been in a position that gave you a first-hand view of what the secretary's post entails, find someone who can talk to you about it, such as last year's secretary or someone else who has served before on the student council.
Specifically, you should learn how thorough the notes the secretary keeps are, what the secretary does with these notes after the meeting, whether the secretary handles other correspondence for the council and what the secretary's role in other council work is.
Show You Can Do It
Understanding what the job really is gives you a definite heads up over your competition in showing your peers that you can do the job well. Particularly in your speech and in an abbreviated way through your posters, articulate the experiences and skills you have that will make you do the job well. Your writing and communication experiences will be the most relevant to your duties taking notes, but your experiences working collaboratively with teams will also be important as a member of the council. If you'll participate in designing projects and voting on decisions on the council, you should explain you'll be a good representative of the student body as well.
A secretary needs to be neat and organized, and your posters are a chance to show your peers that you have these traits. Design a clean and clear poster to put up around the school. You might play up your note-taking skills by designing a poster that looks like the notes you'd take as secretary, except that the subject is why you should be chosen. Getting some friends to help you put up posters will help make sure there are enough up that everyone sees them.
Also think about things you can do as secretary that others wouldn't be able to do or wouldn't think to offer and share these ideas with your peers. Maybe you can create a website where notes from council meetings are posted for the student body to see or work on behalf of the council to contact interesting people who might come speak at the school.