There are two ways to run such a thing.
In either case you need to make sure that the numbers chosen are really random and that nobody can know them in advance. Thinking up some numbers and telling an admin has worked in the past, but is open to abuse or, at least, mistrust, as players have no way to know who else you might have told. The only way I know to make number selection completely above board is to chose something that cannot be determined in advance and that everyone else can look up to verify the winning number(s).
Some examples of this are state-run lotteries, prices of publicly-traded commodities, and sports scores.
Carefully work out all the rules in advance (and try to think of all the ways someone could follow the letter of the rules while going against the intent). If you try to change any of the rules once the game has started people will lose faith in you and will not play. (In the worst case, they will accuse you of scamming which could get you reset).
Think about
You'll need a very high jackpot and/or give away great items if you want to interest more than a couple of people. Also, expect flamewars to erupt in your threads as some players try to convince everyone else that your game is stupid and unfair. Be prepared to clearly explain why your game is fair and have numbers to back it up. (don't be afraid to ask for help from admins if some players become abusive)
You can see some previous attempts at running lotteries here. Personally, I have no problem with you running a lotto and wish you the best of luck with it.
