Please don't download Final Draft off Frostwire (which is an open source Limewire client) or any other filesharing service guys. That's software piracy.
Puppetwise, you can use Word to write scripts but it takes a lot of hassle to keep them in the proper format. The best thing to do is use Final Draft as Faze suggested (just buy it, don't download it illegally). There's a free demo available from
http://www.finaldraft.com that you can try out.
If you wanted to get Final Draft, a really good idea for you since you're 15 might be to contact the Australian Writers' Guild (AWG), tell them you're a student interested in writing scripts and see if they have any special discounts or programs.
If you can't afford Final Draft, there is a free script writing program called Script Maker available at
http://www.mindstarprods.com/cinergy/ScriptEditor.html and you can find others if you take the time to search a little with Google.
One thing you should do sooner rather than later is learn to write in proper script format if you don't know how to already. If you eventually want to get in to film or TV (or theatre for that matter) no one will take you seriously unless you're able to write in the proper formats so the sooner you get in the habit the better. The nice thing about script writing software is that it does most of the work for you!
A really excellent book I'd recommend reading is Elements of Style for Screenwriters by Paul Argentini. If you check out your local library they probably have at least one or two books on writing scripts and screenplays.
Keep in mind too that Australia has a slightly different layout for scripts than the U.S. There's a good guide to Australian script layout located at
http://www.afc.gov.au/downloads/pubs/sugg_script.pdf