Most good trackers can be used completely free of charge. But you have to work your way up to them, proving yourself as worthy on a lower tier tracker.

There are plenty of guides on this. Most recommend starting with an open interview on RED or MAM, then using the invite forums once you qualify.

You can also watch for open sign-ups. TL opens its doors a few times per year (and usually during December), and it's at least as good as the public trackers. Sometimes these have invite forums, but usually not. TL does not.

You will, however, want to spend money on a VPN with port forwarding. This should run ~$30/year.