* Anonymity is vitally important, especially since you are an inexperienced programmer with a high risk of introducing security holes. Don't collect any personal details (including email addresses) that could cause problems for people if they were leaked. Don't implement Facebook or Google signin!
* If there are any social components at all, consider the potential impact of trolls. This project may require 24/7 moderation.
* This is a major emotional commitment. Talking down suicidal strangers is not something to go in to lightly! Make sure you have a professional advisory network in place.
Since lots of people have offered to help, I suggest getting them added to a mailing list ASAP while they are interested.
You could consider building the project in public on GitHub - that would allow technical advisors to review your code for you and use the issue tracker to discuss features.