Please see the download page for details about how to obtain, compile and run trojita.
The most comprehensive reading about trojita is undoubtly the 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.
If you encounter a bug in Trojitá, please report it to it's issue tracker. Wishlist items belong to the same place.
Trojitá's IRC channel #trojita (connect from browser) is hosted on Freenode network. The main author, jkt, is often there.
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. You can also of course mail the author directly.