Do you have firewire on your PC? (Most low/mid range PC's do not come with firewire standard.) If not, look into getting a firewire PCI expansion card. Most cards come with free editing software, usally Ulead Video Studio, or Pinnacle Studio. Both are great for starting film editors. In fact, I myself started with an old version of Ulead Video Studio (that came with my firewire card). Another great thing about the free versions of the software is that you can upgrade to the latest version for
very cheap. In addition, many of these companies have "Pro" versions of their software, for example Ulead has Media Studio Pro while Pinnacle has Liquid Edition. If you ever decide to move to a more robust program, these upgrade paths are always very easily available for very little money.
Example:
Let's say you buy this firewire expansion card:
http://www.newegg.com/app/ViewProductDesc.asp?description=15-124-003&depa=1
That comes out to 12 dollars plus 1 dollar shipping. Install the card and install the included software (Ulead Studio 5.0). Already you've got a very nice editing machine.
Let's say 2 months down the road you decide to get serious into filmmaking and want a more robust program so that you can complete your films faster, easier, and with more effects. Well, you can upgrade Video Studio 5 to the fully featured and quite powerful (and underrated) Media Studio Pro 7, which would set you back around 200 dollars.
My point is that you could get fully featured software for (relitively) very little money. (for $215 you can get a 500 dollar program!)
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On the other hand, if you already have firewire, using free programs like Movie Maker and FreeDV are just fine for starting out. If you ever feel the need to advance to a better editing program, remember that because you are in high school, you can purchase academic liscenses, as
Lazlo mentioned. If you go this route, I would also recomend Sony Vegas, for it's superior audio tools, and also because of it's user freindly UI.