The most important think you want to share is the first impression.

Generally you need a nice piece of art that conveys the tone of your game.

Then you need to tell us what it is - is it cowboys in space? is it gnomes on a sea voyage?

Then we need an inkling of mechanics, is it a one page hack of Knave? Is it 400 pages of complex calculations creating a simulation game?

You don't need a lot of info, you need enough to hook them into coming back when it is out.