supposing your game is working and has been tested and all that;what''s the next step?where do you go to market and sell products.if smaller demo versions are posted on the net,is there any chance they''d be noticed by some big commercial houses or would it be better to go it alone??

first, this belongs to the business section.

If you have a game which is tested and good enough you approach publishers directly. Usually you start talking to potential publishers early on, before you even start creating any content. Most publishers like to look over the GD and make small (or big) changes before they´ll even consider comitting any resources to a project.
If your game is finished and tested you would start contacting publishers ASAP, sending them a brief description first, then (if they are interested) the full game. There´ll be an NDA of course, no publisher will accept any submissions without one (at least those who are serious about their business).
As for publishing by yourself, I recommend against it. Only very few companies have the resources and connections to do their own publishing.

I agree with Jester.

Personally, I would recommend going the Shareware route.
This is not because publishers are bad, but I think Hase covered it quite nicely.

With your next game, and if you want to go through a publisher.. having your shareware game under your belt (and hopefully with sucessful results), your chances will/should be increased dramatically.

Of course like everything in life, this may not always be the case. For example, Blizzard are pretty "pickey" about what they produce, same with other big houses like ID, and Bioware.
Sierra isn''t too bad and neither is EA (how ever everyone has stories about every publisher if you do a search on here for them).