Hello Vignesh
there are two ways that you can vary speed of AC motor
1) Variable Frequency Variable Voltage – used in VFD – variable frequency drives
2) Phase angle control – used in domestic fan regulators
for 1st option, you can get PWM chips or controller readily available. just search for it on google.
for second option refer my following article
http://www.engineersgarage.com/contribution/ambhatt/single-phase-ac-motor-speed-controller
it gives you much better idea