How do I start on building a newsletter on Rails?

I want people to click subscribe w/ email so I can inform them when my app is launching soon.

As discussed in the chat, the easiest way seems to use a hosted newsletter service such as MailChimp and just embed their signup form. No need to custom build something for Rails, unless you need deeper integration.

Also, WIP now has a prototype available for signup forms for your products. For example:…

It's not possible to embed it on your own site, or host it on your own domain. But let me know what you'd like and I might add it.

Another option is PH's Ship which adds social proof and 1 click subscribe for Product Hunt users. Might be a good fit if you want to launch there, and you can export the list to Mailchimp to send your own emails.