Backlash is caused when the spool spins faster than the line coming off of the reel. To prevent this, you need to set the spool tension knob (located near the handle) and your reel's braking system to the right setting for the weight of the lure you're using.
First, turn your braking system to the off or free spool setting. Then hold the rod at a 45o angle, and with the tension knob tightened, slowly back off the tension until your lure starts to slowly drop. You want the spool to stop spinning as soon as the lure hits the ground.
Next, adjust the braking system. Begin with the maximum amount of braking dialed up, then gradually reduce from there until you are able to reach the distances you need without causing a backlash.
Always remember, casting with a baitcast reel takes practice to make perfect.