I have a website and I'm 14 years old.How can I advertise my comic without having to spend any money?

weeeelllll that's a bit of a tough thing to ask. obviously the most effective way of free advertisement is word of mouth. You could make some online friends and do link exchanges with em. then there's posting in forums, linking to your comic in your signature... Ya gotta use your imagination really!

With massive difficulty.
Getting people to read your comic is like getting blood from a stone sometimes.

Yes. A few things to do.

First, every time you post in a forum and your signature contains a banner for your comic? That's an advertisement. So, be active in the forums you're in.

Second, you can look into Project Wonderful ads. Typically, people earn a little money publishing ads, then spend that money on ads.

Third, make sure you're listed on every webcomic listing you can find, Thewebcomiclist.com, frumph.net, belfrycomics.net, topwebcomics.com and others.

Join banner exchanges!

Also join this:
http://onlinecomics.net
It's where many people go to find new comics.

I'm sad because OC.net barely works half the time now.
Its creator(s) run off into other comic endeavors, leave one of their greatest creations (and my favorite webcomic listing website) to rot for over a year.