Hacktoberfest is open to everyone who owns a github account. Pull requests can be made in any GitHub-hosted repositories/projects.

Participants can sign up anytime between October 1 and October 31.
To do so, navigate to https://hacktoberfest.digitalocean.com/sign_up/register.

To get a shirt, one must make five pull requests between October 1 and 31. that can be to any public repo on GitHub.
The pull requests must contain commits you made yourself.
This year, the first 50,000 of the participants can earn a T-shirt.

You can check your progress by navigating to https://hacktoberfest.digitalocean.com/stats/

This year's shirt looks like this:

Hacktoberfest T-Shirt

Hacktoberfest is presented by Digitalocean, Github and Twilio.