You need to know what you want to do really. Just saying a computer related job is too wide an area. The problem is you're only 12 (according to your profile) so you could decide on an area that won't even exist by the time you're old enough and qualified to move into that field.
The most important thing to have these days is commercial experience, employers are rarely interested in people with xyz degree in something fancy, they want at least 1 or 2 years experience in xyz.
If you don't already then start looking at how hardware goes together, how OS's work, how networking works i.e. the basics. Get some web design under your belt, offer to fix other people's machines, keep abreast of new technology so that if anyone asks you know what the next generation of cpu is without having to google it. Linux is big in the world of servers and is gaining popularity on Desktops so again if you aren't into it already then start downloading some free distros and having a play.