when I start the car, the Tachometer goes up to 1

Hi,
I just got Ford Falcon au 2002 duel fuel, it runs perfectly, but the problem is, when I start the car, the Tachometer goes up to 1 but when I change the gear from P to R or D, you start feeling that car is gonna stop, but it doesn't, and Tachometer goes to under 1 by 3 lines, then it goes up to 1 again, so how could i fix it ???
Thanks