Welcome to the wonderful world of web design! It's really essential to test your site as you go in as many browsers as possible to spot problems as they arise. It's a nightmare to fix things when there are multiple problems built atop each other.
As webdev says, a good place to start is validation, but that won't pick up 'valid' but unworkable page layout techniques. We can try to help you if you post a link, but if all else fails, I'd suggest setting up a test page where you strip the code down and build it up again step by step, testing in each browser as you go. If you haven't tested in Internet Explorer yet, you've got even more pain to come.