either:
a) Use browser hacks to 'cover up' the differences
or
b) Be very specific about all tags, elements etc, and avoid those which are rendered differently
Clearly the second option is more desirable.
See such sites as:
http://www.netmechanic.com/products/Browser-Tutorial.shtml
http://blazonry.com/info/difftable.php
http://www.quirksmode.org/css/contents.html (the best comparison table imo)
and search for the tags you use and what differences they may have.
Perhaps explain better what you mean by 'differently', it is possible that you haven't yet acquired the coding knowledge to produce robust code (browsers do funny things). Check out things like DOCTYPE and be very careful with TABLE (avoid if possible), position:absolute; (avoid like the plague), and make sure you are writing what you declare (be it html, xhtml...)
If still in difficulties, post a snippet of the problem area (or point us to your url)