How to make my menu responsive

I've created one navigation bar and i want it to be responsive but i don't know how. Can anyone help me ?

Here is the code : CSS:

          .fixed-nav-bar {
      position: relative;
      z-index: 9999;
      width:100%;
      height:70px;
      border-bottom:4px solid red;
      background-color: Tomato;
    }
    a {
    font-family:'Montserrat',serif;
    font-size:25px;
    color:white;
    text-decoration:none;
    margin-top:40px;
    }
    a:hover {
    font-family:'Montserrat',serif;
    font-size:25px;
    color:coral;
    text-decoration:none;
    }
    a:active {
    font-family:'Montserrat',serif;
    font-size:25px;
    color:Beige;
    text-decoration:none;
    }