looks like you're either changing the width of a column so it no longer fits or you are putting in too big of images and its expanding.
CSS is pretty easy, but you can't just increase the width of a div if it has another floats next to it and they have a parent div with a set width otherwise you'll get results like that. Also if you start changing your CSS be sure to make sure you stay valid by validating it at
http://jigsaw.w3.org/css-validator/