Mary,
To second Cathy's suggestion and to add a tip.
You can practice barre chords a little easier by using a capo too. But as Cathy said you shouldn't have any buzzing.
Now don't confuse fret buzzing with the strings buzzing because you're not pushing hard enough!
Don't be discouraged. Barre chords are harder on an acoustic guitar than an electric. Don't go razzdrazz because you see electric guitar players move through them so effortlessly. Also make sure you've warmed up your hands by playing for a few minutes before you start work on barre chords.
As Cathy suggests practice switching from a barre to a "regular" chord and back again.
You'll get 'em.
When you do, you can try some on your 12-string!