HOW DO I POST A SHORT DEMO OF MY GAME ONLINE ?

First you'll want to create a game page. To do so, hover over the Submissions bar at the top of the page and click on Submit Game. Follow the directions given on the loaded page and add three decent, different screenshots as well as a good description of a decent length.
You then have to wait for the Submissions mod to accept it (if it doesn't pass he'll send you a PM telling you what to change) then once accepted you hover over your name in the top bar and choose Manage Games. Pick the game profile, then choose the Downloads tab and add either an .exe or zipped file.
It may take a while depending on the size of your game.

If it's only a small game you could upload via your locker space instead (while waiting on Game Page approval). Hover over your name and choose the Locker option. It should tell you on the page how much space you have in your locker. (The space will change depending on the amount of MS (Maker Score) you have. You can get more MS by adding to the site - reviews, media, games and the like will give you points. Also winning competitions and gaining achievements.)

Choose to upload to locker and again, pick the file (zipped or exe) that you want to add.

If you need further clarification just ask.