Bar chords are difficult to play at first and require a great deal of practice before they are comfortable to use. Try to keep the first finger of the left hand straight and parallel to the fret wires. It is also important to keep the other fingers arched and use the tips of the fingers only. The left hand thumb must be kept behind the neck.
Once you are reasonably comfortable with the full bar technique, try playing an open E chord with your 2nd, 3rd and 4th fingers and then sliding them up one fret and placing a full first finger bar behind them to play an F chord. This is demonstrated in the following example. The G and Am chords here are both open chords.
You are often required to move quickly between single notes and bar chords on the electric guitar. In the following example, all the single notes are played with either the first finger or open strings.
The Bm chord uses a bar across 5 strings and the A chord is played with a first finger bar across the 2nd, 3rd and 4th strings. This one should give your first finger a good workout. Notice that the first string is not strummed in this example.
Bm
A