How to make sidebar float right and site content left

You would do something like this with your CSS. Setting a fixed height and whatnot is only for example purposes:

#content {
  background: brown;
  width: 75%;
  height: 500px;
  float: left;
}

#sidebar {
  background: gray;
  width: 25%;
  height: 500px;
  float: left;
}

<div id="content"></div>
<div id="sidebar"></div>