I heard about an official green card lottery in U.S., can you tell me the real web-site of this lottery?

The DV lottery is free but there are several websites out there that charge to file and application and try to appear to be the official site. The State Department website has a good discussion of the program:

http://travel.state.gov/visa/immigrants/types/types_1322.html

and the official site for the DV program is here but the program is closed for this year:

http://www.dvlottery.state.gov/