To begin with, most car wheels are centered when the steering wheel is straight. If you have a three spoke steering wheel for example, one of the three should be pointing down.
Now, as you turn your wheel, this "position" might occur again after one revolution, but steering wheels usually only turn about 1.75 times in each direction, so that "position" will only occur 3 times (try in your car to be sure...).
So if you turn your steering wheel all the way to the left, and then back until the spoke is pointing down, and then one more revolution, your wheels should be centred.
If for some reason your steering wheel happens to be not this way, you can also try to turn it all the way to the left, then all the way to the right, and remember how many revolutions you had, and turn half of that amount back (e.g. you turn it 3.5 times from full left to full right, and then 1.75 times back to centre the wheels).
By the way, when parking on hills you might want to turn your wheels purposely as a kind of hand brake. If you turn your wheels, so that if the car would start rolling downhill (for what ever reason), the front wheels would hit the curb, you can assure that it won't roll away, even if your handbrake fails or is not working.