I am creating a page with a custom fields that I am concerned people will not want to fill out unless the page is secure like a donation page. I tried making it on a donation page but I didn't want the donation features. Can I make like a petition page secure somehow?
You'll want to consult "What are NationBuilder's SSL offerings for websites?" about overall SSL with NationBuilder.
Pages connected to a custom domain will not feature an SSL certificate and display as http.
Complete and current information on how to use NationBuilder is available in the documentation section.