@Loussine , first off. I would advise providing a link to your project for easier debugging for us.
Now coming to your initial question, you can use media queries to make your webpage more responsive to change in screen size. Bootstrap provides an easier way to use media queries, you can try that too.
To top it off, you should consider using css preprocessors, they usually add some extra stuff to your code to make it more browser responsive.
Finally, consider cross browser testing to make sure you have it all done right.
Check out this webpage for more…