how have two parent div next to each other - css

Question

I'm working on a css/html site and I have this problem: in my style.css I got two divs, one for sidebar and one for content. I would like the sidebar to be, well, at the side, but when both are present the content always goes below the sidebar, unless I put that in position:absolute. In that case I have a problem when small screens are used, 'cause they get one over the other.
I paste the code, hope you can help!
/* This is for content */
article, aside, figure, footer, header, hgroup, menu, nav, section {
display: block;
}
#modal {
position: relative;
width: 100%;
display: inline-block;
display: table;
display: table-cell;
float: none;
border: 1px solid #CCC;
box-shadow: 0 1px 5px #CCC;
border-radius: 5px;
font-family: verdana;
margin: 25px auto;
text align: center;
background-color: #f2f2f2;
overflow: auto;
}
#modal header {
background: #f1f1f1;
background-image: -webkit-linear-gradient( top, #f1f1f1, #CCC );
background-image: -ms-linear-gradient( top, #f1f1f1, #CCC );
background-image: -moz-linear-gradient( top, #f1f1f1, #CCC );
background-image: -o-linear-gradient( top, #f1f1f1, #CCC );
box-shadow: 0 1px 2px #888;
padding: 10px;
}
#modal h1 {
padding: 0;
margin: 0;
font-size: 16px;
font-weight: normal;
text-shadow: 0 1px 2px white;
color: #323232;
text-align: center;
}
#modal section {
padding-right: 10px;
padding-left: 10px;
padding-bottom: 10px;
font-size: 12px;
line-height: 175%;
color: #333;
text-align: center;
}
while this is the sidebar
/*sidebar */
#sidebar{
position: static;
margin-top: 15px;
max-width: 150px;
}
#sidebar ul,
#sidebar ul li,
#sidebar ul ul {
list-style: none;
margin: 0;
padding: 0;
border: 0;
top: 5%;
}
#sidebar ul {
position: static;
top: 90px;
z-index: 1000;
}
#sidebar ul li {
min-height: 1px;
line-height: 1em;
vertical-align: middle;
}
#sidebar ul li.hover,
#sidebar ul li:hover {
position: relative;
z-index: 599;
cursor: default;
}
#sidebar ul ul {
margin-top: 1px;
visibility: hidden;
position: absolute;
top: 1px;
left: 99%;
z-index: 598;
width: 100%;
}
#sidebar ul ul li {
float: none;
}
#sidebar ul ul ul {
top: 1px;
left: 99%;
}
#sidebar ul li:hover > ul {
visibility: visible;
}
#sidebar ul li {
float: none;
}
#sidebar ul ul li {
font-weight: normal;
}
/* Custom CSS Styles */
#sidebar {
font-family: 'Lato', sans-serif;
font-size: 18px;
width: 180px;
}
#sidebar ul a,
#sidebar ul a:link,
#sidebar ul a:visited {
display: block;
color: #000000;
text-decoration: none;
font-weight: 300;
}
#sidebar > ul {
float: none;
}
#sidebar ul {
background: #a0a0a0;
}
#sidebar > ul > li {
border-left: 3px solid #000000;
}
#sidebar > ul > li > a {
padding: 10px 20px;
}
#sidebar > ul > li:hover {
border-left: 3px solid #9b1a1c;
}
#sidebar ul li:hover > a {
color: #9b1a1c;
}
#sidebar > ul > li:hover {
background: #f6f6f6;
}
/* Sub Menu */
#sidebar ul ul a:link,
#sidebar ul ul a:visited {
font-weight: 400;
font-size: 14px;
}
#sidebar ul ul {
width: 180px;
background: none;
border-left: 20px solid transparent;
}
#sidebar ul ul a {
padding: 8px 0;
border-bottom: 1px solid #eeeeee;
}
#sidebar ul ul li {
padding: 0 20px;
background: #fff;
}
#sidebar ul ul li:last-child {
border-bottom: 3px solid #000000;
padding-bottom: 10px;
}
#sidebar ul ul li:first-child {
padding-top: 10px;
}
#sidebar ul ul li:last-child > a {
border-bottom: none;
}
#sidebar ul ul li:first-child:after {
content: '';
display: block;
width: 0;
height: 0;
position: absolute;
left: -20px;
top: 13px;
border-left: 10px solid transparent;
border-right: 10px solid #fff;
border-bottom: 10px solid transparent;
border-top: 10px solid transparent;
}