<br> and top-margin

How should I use the <br> tag in xhtml.
I get errors if I only use <br>. If I change it to <br/> then the errors are
gone. Is <br/> a legal tag or attribute.

Where should I put the top-margin="0".
If I put it in the body tag I get errors. 
Does netscape accept this? If not what should use then?

Thanks
Regards
Deon