My system is not really a powerful one and so before installing any game I would really like to know about the requirements of the game I'm playing 
but I can't find anything like that anywhere 
where ca I find the min spec requirements?

On occasion, they're listed on the product page. Possibly with some of the info in tags as well.

On other occasions... they aren't.

And if it's a web game.... while it's not guaranteed, chances are it will probably run.

Chances are, if your rig can run your operating system and the browser you used to type this comment, it can run about 99% of the games here (that are marked for your operating system)

There are some 3d games here, that might have some kind of requirements. And maybe some types that have driver requirements. Try to install a demo version, if a game has any, before buying. Also, games that might have problems here, tend to list the requirements. 

They should be on the game page, but it's up to the creator to fill in this information. Leave them a comment or something.