To play a bar chord, you'll need to use your index finger to hold down all 6 strings at the same time. This is the 'bar' of the bar chord. Your other fingers will play the different notes of the bar chord so get used to various shapes.
First get accommodated to how each bar chord feels. Then practice moving from one bar chord to another.
Play a song that is played in all bar chords (a jack johnson song for example). Your thumb will get too tired to play the whole song, as holding down a bar chord requires a lot of pressure from your thumbs. Don't worry, the more you practice the stronger your thumb muscle will become.
Expose yourself to all different kinds of bar chords and develop enough finger strength to easily hold the shape of the chords as you move up and down the strings.
When you first start out, you'll experience trouble barring the strings. You'll hear string buzzing and string muting where your fingers aren't strong enough to completely push a string down. This can only be solved with time spent developing finger strength.
Once you get the bar chords down, experiment with different plucking and strumming patterns. The barring of the strings essentially acts as a moveable capo, allowing you to play some unique melodies.