Please see the download page for details about how to get Trojitá.
The most comprehensive reading about trojita is undoubtly Jan's Msc. thesis. It discusses design of the application and provides a comparison with various other mail user agents.
A series of blog posts about Trojitá might provide an interesting insight into its background, too.
Bug tracking happens in KDE Bugzilla (see list of open bugreports). If you encounter any problem in Trojitá, please file a bug. Wishlist items belong to the same place.
Previously, Trojitá was using Redmine for bug tracking, and some of the bugs are still tracked in the old tracker.
Trojitá's IRC channel #trojita is hosted on the Libera network.
There's a mailing list at trojita@lists.flaska.net, archived by GMane. To subscribe, write a mail to trojita+subscribe@lists.flaska.net, posts from addresses which are not subscribed have to wait for moderation. The issue tracker or the mailing list is the preferred way for reporting bugs and requesting features, while the IRC channel works well for general questions.