In my wepage(made with dreamweaver),i discover that there is some space on top of the banner section and the background colour shows above despite setting all margins to zero.What could be the cause of this and the solution.

Have you set all the paddings to zero as well? If that isn't the cause, it could be caused by about a thousand different things, so without the actual code (html & css) or better yet an online test version we can't really help you.

this could be caused by other float clearing, have you check other elements that may affect it.

It's all a guessing game unless you provide a link to the page we can take a look at... :-)

Nadia