My website looks good in internet explorer but it looks funny in firefox and chrome. What are the differences in the coding/css for all three? How do I fix this problem?

One way we learned in one of my web dev class is to use javascript to detect the specific browser as well as version and then use a selection structure (if / elseif / else) to check whether it's Internet Explorer Version x, Firefox, then set up the page like so, etc. We'd utilize the navigator class to make use of detecting the browser name and version - check it out http://krook.org/jsdom/navigator.html

I'll back secureseven in saying stick to w3 standards. It's good they give a ton of examples and show what browsers they're compatible with - you could take this into consideration when developing.