A barre chord is an open E chord with your index finger acting as a movable nut. Wherever you put it, the note under your index finger on the low E string names the chord.
At the 1st fret it's F major. Lay your index finger flat across all six strings at fret 1. Then form the E shape with the other fingers: ring finger on the A string and pinky on the D string, both at fret 3, and middle finger on the G string at fret 2.
The index only needs to press firmly on the low E, B and high e strings; the other fingers cover the rest. Roll the index slightly onto its bony side, keep it close to the fret wire, and let your arm's weight pull the neck back rather than squeezing with your thumb.
Pick each string to find the dead ones and adjust. Then play A-G-F-G (frets 5, 3, 1, 3), a classic rock progression. It should sound full and bright.
The most common mistake is squeezing hard with the thumb, which tires the hand in seconds. Use only as much pressure as it takes, and rest when it aches.
