Initially, the best audience for your petition will be your network of friends and family. Tell them why you've created the petition and why it's important. Ask them to sign and to tell others about it. Then, as you collect more signatures, you will want to start thinking more broadly. If it's a local issue, reach out to people in your community. For other issues, connect to groups that have an interest or stake in the issue.
It is very important to remember that you are not the only person who cares about your issue. Each person who signs your petition and reads your updates is a potential ally. Help them become advocates. Make sure the information you post is accurate and up- to-date. Don't overwhelm the petition with lots of dry facts, but include a link where people can go for additional information and downloadable resources. The more people you can recruit to help—even informally—the more opportunities there will be for others to hear about your issue.