Analog IN Problems

Hi, I've purchased an Arduino uno board, but when I tested it's analog input it says 0-1023 values, there's nothing attached into the board.
For example:
0
234
1023
450
888
777
678
0
1023
1023
1023
...
Isn't it supposed to say 0 when the voltaje is 0v?
What can I do?
Thanks!

When "there's nothing attached into the board" the pin is floating - the inputs need such little current to change the voltage level that the readings can be all over the place as you've seen.
Put a 10K to 100K resistor from the pin to Gnd, and it will read more like 0.

Thanks! It worked =)