It will be easier for you to use a plugin to do what you want.

Here is a Google search will several results which can help you,

https://www.google.ca/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=wordpress%20responsive%20video%20embed

Here is a plugin you can use https://en-ca.wordpress.org/plugins/advanced-responsive-video-embedder/