The bridge shown in the Photo is a les paul type - it's string length adjustment is done by turning the slotted screw at he back of the bridge.
OR if it's a stratocaster type bridge the length adjusment is a philips screw at the back of the bridge.
Turning these screws will alter the tension of the string, so it's a good idea to slacken the strings at the machine heads before adjusting.
Some guitarists can set them up by ear, but I always need a tuner - further back in this thread, is an explanation of how to use the tuner - by playing an open string and then playing the same note an octave higher on the 12th fret