The currently untitled film's script will be written by Craig Schulz (Charles Schulz's son), Bryan Schulz (Craig Schulz's son), and Cornelius Uliano (writer of several short films). No plot details have been announced yet.
Directing the film will be Steve Martino, who has directed two other films for Blue Sky Studios: the 2008 feature film version of Dr. Seuss's Horton Hears a Who! and the 2012 film Ice Age: Continental Drift.
You can read the official press release at Blue Sky Studio's website.
While the press release doesn't explicitly state how the movie will be animated, reading between the lines suggests it will be computer animation of some form (especially since all of Blue Sky Studio's films so far have been CGI).
Keep in mind that the film has just been announced and isn't due for another three years; the details are probably still being worked out and films often undergo a variety of changes as they are produced. For example, the film's planned release date changed only six weeks after the initial announcement - initially they said it would be released November 25, 2015. It's likely there will be more changes to the release date over time; so if the date listed in this article doesn't match other dates you see, it probably changed again.
This isn't the first Peanuts theatrical film; as fans will know, there were four earlier theatrically released films - A Boy Named Charlie Brown (1969), Snoopy Come Home (1972), Race for Your Life, Charlie Brown (1976), and Bon Voyage, Charlie Brown (1980).
Stay tuned for more details in the months ahead! But in the meantime remember there are lots of other great Peanuts animation shows and films to enjoy, from the classic holiday specials, to the four earlier theatrical films, to the most recent release, 2011's Happiness is a Warm Blanket, Charlie Brown.