While you are still in High School, I would recommend you do some reading, studying and practice on networking. There are lots of books that you can buy online and Google to help you out, and the more you can learn now the better it will be for you later, when hunting for a job.
Employers are often looking for candidates who hasn't only got certifications but experience too. So I would start by getting to know what network routers and switches are, how they work as well as learning all the networking protocols, such as HTTP, FTP, RIP, IGRP, ARP, DNS and etc.
On the other hand, you will also need to learn and have a good understanding of how IP addresses and subnetting work, very important to know when building or managing a network.