my horrible css code has broken my site

Hello I just finished publishing a site for a gym, the thing is it looks great on my computer which is 1366×768 but on every other screen size my header/background go away and my top menu items float away. My css isnt that awesome I’m print designer transitioning over. I just need some pointers on how to keep this site from breaking. the link is https://ironforceathletics.com also im using the canvas theme by wooThemes, thanks in advance