Both at once. Learn your scales, starting with G, D, A, and C. Practice `em. Get a chord book to start with, but learn the notes that make up the chords and work out several variations. If you're going to play bluegrass, learn the full stopped versions (fretting all strings) so you can do chop chords.

Above all, have fun with it. It's a great instrument.

And check out mandolincafe.com.