Get Good Racing Starts on a Dirt Bike

The standard way to start a race with a little twist.

Steps

Lean forward. This will push your weight towards the front of the bike so when the race begins you move faster than when the weight is towards the back of the bike. This step will be easier if you sit towards the front of the bike.
Put your feet in front of the pegs.
Hold on tight. You would not want to fly off and get dirty and bloody and hurt would you.
Let out the clutch fast and hammer the gas.
When you bring your foot up to the peg, shift up.

If the start lane is short, shifting is your own choice. Shifting in short start lanes can make the engine bog down at the corner.
Hold the clutch at a position right before the gears catch - slowly let out the clutch and when the bike starts to move, pull it in just enough to take away the power.
If you are riding a 2 stroke, let off the gas a little (keeping in the power band) to get better traction.
How much you lean forward depends on the amount of traction. If the dirt is harder, lean further forward. If it's softer, sit straight up more.

Every part of this is important for success. Excessively high wheelies can and probably will happen for beginners unless all the steps are followed perfectly.