frequency changer

i am using 555 timer to generate frequency thought i dont know how to change or control frequency though i could use variable resitser but i think that too big, so I also dont know how to increase the cycle/s frequency or does it matter cauz i want at leat 30khz of ultra sonic sound.

Sometimes the datasheets for devices contain applications information that can be of great help. Look at National's sheet for an LM555 to see if that helps.