Barre Chords are notoriously difficult, but you'll find it easier as the strength of your hand increases.  Barre chords - or as some call them, bar chords - are called that because the index finger is held across all the strings, hence they are ?barred'. Just to make it even more confusing, the name of the chord changes as you move your fret and string positions.

When practicing Barre chord positions you will need to strengthen your first or index finger considerably. The side of the index finger that is closest to the thumb is the part that should lie across all six strings in a Barre chord position. To practice, place your index finger across all six strings at the 2nd fret and strum. Use your thumb to hold that index finger down. It can take up to six months to build up enough strength to do this successfully, so be patient. Once you succeed, then go on to learning Barre chords.

When starting Barre chords, rest the guitar on your left knee and hold the neck at an angle of 45%. This position will help you to get your hand in the correct position for getting that index finger strongly across all six strings. Don't wrap your thumb around the neck, but rather push it across the back. Imagine that you are pinching the neck between your finger and thumb, while still keeping your index finger flat on those strings.
 
Don't forget to use the large knuckle at the base of the index finger when using it to Barre with. There is strength in that knuckle and it can be used to fret the first and second strings. This will make it all a great deal easier. It's minor adjustments like this that make the difference between success and failure, pain and pleasure.

For a Major Root 6 Barre Chord, bar all the strings with your index or first finger, 2nd finger on G-string, third and fourth fingers should be on D and A strings. All the strings should ring clear but until your hand strength builds up they may not. Simply remove your finger from the G-string to get a Minor Root Barre 6 Chord.

When you get into it, Barre chords are really simple and easy to remember. You need only learn three forms to be able to work out many others because you simply move frets. Downloading a free diagram of the chords is the best way to learn them properly as you can then see at a glance where your fingers need to be.