There are numerous problems with IE6 and it's hard to keep track of them all along with their respective solutions. A good rule of thumb is that IE6 often has support for a function, but interprets it in a wrong way. Apart from that, it also has some quirks that can be avoided by resetting the default value. Using "position: relative;" can solve some problems, for instance.
For a list of some common bugs, you can go to
positioniseverything and look it up. On their
main page you can also find lists of bugs in other browsers as well as good ways of solving them.
If you have a specific problem, it can often be solved by asking in this forum. Just include a link to the site and people will most probably look into it within a few hours.