First off let me give you some background on me. I'm currently 17, turning 18 in a month or so. I just graduated from high school and am currently looking for a university.

Congratulations and good luck :-)

<mutter>darn whipper-snapper jack-a-napes springing up everywhere</mutter>

The real question of course, is how do I prove it to my potential employers? My first thought was some sort of open source project that would demonstrate my ability, but I was unsure if this was actually effective, and if so, what project?

Show them your code (speaking as somebody whose been on the employer side of the table a few times :-)

If it's Perl work a good CPAN module or two always impresses. Failing that stick something that you're proud of up on a public web site and add the URL to your resume/CV/whatever.

Also, bear in mind that it's a bugger of a job market at the moment. There are lots of good people with experience out there, so don't get too disheartened if you don't get many responses (although, and this might be my imagination, things seem to have been picking up a bit again recently).

While you're job hunting I'd recommend finding a local firm doing web work and seeing if there is a small project you can do for a month gratis. Or find a charity with a web site that can be improved with a some decent interactive features and volunteer some of your time. Demonstrate that you're doing something in the area you're trying to work in, even if it's not paid employment.

Best of luck.