Learning barre (bar) chords is the first major hurdle beginner guitarists must overcome, and as with everything guitar related, it takes practice and time. I usually recommend learning everything on the steel string acoustic first, since if you can play on that, than you'll be able to play it on electric and nylon string as well.
BUT barre chords are a bit different, since they are really tough at first, and many beginners actually quit when they reach barre chords. So if you get frustrated with it on the acoustic, by all means, practice on your electric or nylon string guitars, it'll be a lot easier, but you may have difficulties holding barre chords afterwards on the steel string acoustic.
Now as for learning barre chords, it's a 2 step process: