Embedding iframe.js

Hi. I need to embed a file iframe.js into my home page. I have tried to do it by manually coding it into the relevant page which works, however the iframe extend over the whole page pushing my other content out of site. I have tried every plugin under the sun and they either dont work or dont load properly. Does anyone have any ideas or code that can help me ? I want the iframe t have a 100% width and a height of 400px.