Barre's seem to be the Holy Grail for most fledgling guitarists... It's something everyone struggles with. I wanted to share my experiences with barre chords hoping that someone can learn from me.
First of all I'd like to disagree with those that say you should focus on your open chords first, and build hand strength, before attempting barre chords. In attempting to barre, you will build up the hand strength. My second reason is that with proper technique, you don't need nearly that much hand strength! I really battled with barre chords in the begin, I know all about the hand cramps!!! But I came accross a tip on this forum that changed barre chords for me. Instead of using a squeezing action with your left hand, release your thumb pressure, and pull back on the neck with your left elbow. At the same time push back on the body of the guitar with your right elbow, as if you trying to wrap the guitar around your body. That action takes alot of pressure off your hand, thumb and wrist as it places the joint into tension as opposed to compression. You can use your thumb to focus some extra pressure to any "dead" sounding spots to make the chord ring true and clear.
I started out with B#m and F#m in a few songs. But quickly have progressed to F, B, Bm, Gm, F# etc. But choose any song you like with one or two barre chords. And then practice till you nail it. Determination is 90% of the battle won. Doesn't matter which barre chord you learn first as the rest will come naturally.
And lastly just play as much as you can! Don't worry if it doesn't sound perfect, that comes in time. Just focus on playing.
I loved the sentiment in an earlier post: "If your fingers don't hurt, practice more!". And I'd like to emphasize this. Mastering barre chords will take considerable time and effort. Stick at it. Guts it out. And practice, practice, PRACTICE!!!