The title of the first spin-off movie will be Rogue One and will star English actress Felicity Jones, who has appeared in many movies including The Amazing Spider-Man 2 and The Theory of Everything. Chris Weitz, screenwriter for the just-released movie Cinderella, replaces Gary Whitta as writer for Rogue One, while Gareth Edwards is still set to direct. The movie will be released December 16, 2016, almost a year after TFA.
Here is a news and rumors feed for Rogue One on IMDB. It gets updated periodically and is full of rumors, so there may be serious spoilers! Still no official word on what the plot of the movie will be, so for now I will subtitle it Further Adventures of the T-47 Pilot Who Got Superseded by Her Wingman and Totally Failed to Find Han and Luke on Hoth.
Image from Glamour magazine UK, from Golden Globes 2015 (click for bigger image).