side menu with drop down menu

Question

I'm trying to create a side menu like the image below.

I want the menu to appear and the link that is active to have a different color. When user clicks on the shop link I want them to be presented with a drop down menu.

html:

<div id="mainSidebar">
 <div id="menu">
  <nav>
     <ul>

      <li><a href="#">SHOP</a></li>
      <li><a href="#">ABOUT</a></li>
      <li><a href="#">CONTACT</a></li>
      <li><a href="#">LEGAL</a></li>
     </ul>
   </nav> 
  </div>    
 </div>

css:

#menu nav ul li  {
font-size: 11px;
top:106px;
list-style-type: none;
text-decoration: none;
color:#ffffff;
line-height: 19px;
}
nav a {
color:rgb(153, 153, 153);
text-decoration: none;
}
#mainSidebar {
display: block;
font-family:arial;
font-size: 11px;
font-stretch: normal;
font-style: normal;
font-variant: normal;
font-weight: normal;
height: 450px;
margin-bottom: 0px;
margin-left: 0px;
margin-right: 0px;
margin-top: 0px;
min-height: 750px;
padding-bottom: 0px;
padding-left: 0px;
padding-right: 0px;
padding-top: 0px;
position: fixed;
text-transform: uppercase;
vertical-align: baseline;   
}
#menu nav ul {
padding: 0px;
line-height: 11.5px;
margin-top: 0px;    
padding-bottom: 5px;
width: 143px;
padding-top: 5px;   
}
#menu nav ul li a:hover{
color: #ffffff;
text-decoration: underline;
}

Answer 1

I decided to go from scratch on this to perhaps give you a different perspective (because why not):

• Using a straight forward HTML structure
• Using transitions for smoother hover events (you can very easily expand this, imagination is mostly your limit).
• Maintaining font integrity.
• Centering the menu on the screen, as described on the picture. You can change this anytime you like easily.
• Using creative liberty to lightly enhance the look/feel.

EDIT Added animation to the underline that runs under the list titles.

html, body {
    background-color: black;
}
.navigation {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    width: 300px;
    margin: 0 auto;

}
.mainmenu, .submenu {
    list-style: none;
    padding: 0;
    margin: 0;
}
.mainmenu a {
    display: block;
    text-decoration: none;
    padding: 10px;
    color: lightgray;
    font-family:arial;
    font-size: 11px;
    font-stretch: normal;
    font-style: normal;
    font-variant: normal;
    font-weight: normal;
}
.mainmenu > li {
    position: relative;
}
.mainmenu > li > .underline {
    position: absolute;
    top: 30px;
    left: 0;
    width: 0%;
    height: 3px;
    background: white;
    transition: all 0.7s;
}
.mainmenu > li:hover > .underline {
    width: 100%;
}
.mainmenu li:hover .submenu {
    display: block;
    max-height: 200px;
}
.submenu a {
    text-align: right;
}
.submenu a:hover {
    color: white;
}
.submenu {
    overflow: hidden;
    max-height: 0;
    -webkit-transition: all 0.5s ease-out;
    transition: all 0.5s ease-out;
}

<nav class="navigation">
    <ul class="mainmenu">
        <li>
            <a href="#">Shop</a>
            <span class="underline"></span>
            <ul class="submenu">
                <li><a href="">T&#8211;SHIRT</a></li>
                <li><a href="">KNIT</a></li>
                <li><a href="">SHIRT</a></li>
                <li><a href="">CUT&amp;SEW</a></li>
                <li><a href="">PANTS</a></li>
                <li><a href="">ACCESSORY</a></li>
            </ul>
        </li>
        <li>
            <a href="#">About</a>
            <span class="underline"></span>
            <ul class="submenu">
                <li><a href="">YOURSELF</a></li>
                <li><a href="">MYSELF</a></li>
                <li><a href="">JUAN</a></li>
            </ul>
        </li>
        <li>
            <a href="#">Contact</a>
            <span class="underline"></span>
            <ul class="submenu">
                <li><a href="">US</a></li>
                <li><a href="">MARS</a></li>
                <li><a href="">EVERYONE</a></li>
                <li><a href="">DOG</a></li>
            </ul>
        </li>
        <li>
            <a href="#">Legal</a>
            <span class="underline"></span>
            <ul class="submenu">
                <li><a href="">OUR PROMISE</a></li>
                <li><a href="">YOUR PROMISE</a></li>
            </ul>
        </li>
    </ul>
</nav>

Got questions? write them on the comments section :)

Answer 2

This seems to be what you're looking for. I only added the first few lines in the CSS.

If you want it to open on click only and not on hover it requires javascript coding.

body {
  background: black;
  color: white;
}

#menu nav > ul > li > ul {
    display: none;
    text-align: right;
}

#menu nav a {
    display: block;  
}

#menu nav > ul > li > a {
    display: block;  
    border-bottom: 3px solid transparent;  
}

#menu nav > ul > li:hover > a {
    border-bottom: 3px solid white;  
}

#menu nav > ul > li:hover > ul {
    display: block;  
}

#menu nav ul li  {
    font-size: 11px;
    top:106px;
    list-style-type: none;
    text-decoration: none;
    color:#ffffff;
    line-height: 19px;
}

nav a {
    color:rgb(153, 153, 153);
    text-decoration: none;
}
#mainSidebar {
    display: block;
    font-family:arial;
    font-size: 11px;
    font-stretch: normal;
    font-style: normal;
    font-variant: normal;
    font-weight: normal;
    height: 450px;
    margin-bottom: 0px;
    margin-left: 0px;
    margin-right: 0px;
    margin-top: 0px;
    min-height: 750px;
    padding-bottom: 0px;
    padding-left: 0px;
    padding-right: 0px;
    padding-top: 0px;
    position: fixed;
    text-transform: uppercase;
    vertical-align: baseline;   
}
#menu nav ul {
    padding: 0px;
    line-height: 11.5px;
    margin-top: 0px;    
    padding-bottom: 5px;
    width: 143px;
    padding-top: 5px;   
}
#menu nav ul li a:hover{
    color: #ffffff;
    /* text-decoration: underline; */
}

<div id="mainSidebar">
 <div id="menu">
  <nav>
     <ul>

      <li>
        <a href="#">SHOP</a>
        <ul>
          <li><a href="#">T-SHIRT</a></li>
          <li><a href="#">KNIT</a></li>
          <li><a href="#">SHIRT</a></li>
          <li><a href="#">PANTS</a></li>
          <li><a href="#">ACCESSORY</a></li>
        </ul>
      </li>
      <li><a href="#">ABOUT</a></li>
      <li><a href="#">CONTACT</a></li>
      <li><a href="#">LEGAL</a></li>
     </ul>
   </nav> 
  </div>    
 </div>

If you want a click-only solution then remove this from CSS:

#menu nav > ul > li:hover > ul {
    display: block;  
}

and add javascript code (requires jQuery):

$('#menu nav > ul > li > a').on('click', function() {
    $(this).siblings('ul').toggle();
});