I got some positive comments about my project, what now?

Ideally when you're running a test, you want to set it up with an objective in mind from the start. That way you know when it's over and how it turned out.

At this point, you've successfully tested whether or not people in a particular subreddit would be excited about a landing page and sign up for a mailing list. Is that meaningful? It depends what your step 2 is. Usually you want to test the successive links in the chain between where you are now and where you want your business to be, which means you need to have some idea of what the next link is.

For example, with Indie Hackers, my chain/roadmap was roughly: learn what my audience considers compelling content --> figure out how to work with people to produce that content --> promote the content in a big channel like Hacker News --> get people to subscribe to the newsletter so I can reach them indefinitely --> grow through WoM and repeatedly reaching the top of HN. So these were the things I needed to validate and ensure they would work.

Note that the whole point is to save yourself time. Obviously the best validation is to just do what you're trying to do and see if it works. But sometimes what you're trying to do requires months of coding, and if you can get to the answer in a quicker way, you'd be remiss not to.

There are many tools in this validation toolbox:

• finding and talking to customers (I recommend the techniques described in The Mom Test, otherwise you're just going to end up with misleading and meaningless feedback or encouragement)
• eavesdropping on what customers are saying to each other, e.g. by reading their conversations on forums
• analyzing what customers are already doing or buying, as well as how many of them are doing that
• building the smallest possible version of your product, e.g. a landing page, then an MVP
• content marketing in your intended distribution channels to see if they'll work before you build something (usually you want to connect this to some sort of mailing list signup form to ensure people are interested in your forthcoming product not just your content)
• running ads or submitting posts to your intended distribution channels
• etc.

You've got the right idea about sending a "Thanks for Subscribing" email - this will let the people who signed up know that you're a real human. Don't worry about making it professional, make it personal.

Assuming this is a real and legit product that you're working on, continue working on it and document the changes as you develop new features and use this as the content of your update emails.

If you've got a list of emails of people who seem to be super interested in your idea, send surveys to gather as much information on their needs and pain points as possible. use this feedback to understand your market and also to inform your product development. I would use typeform.com or google forms to make something super quick.