How to put two divs side by side

Question

So I\'m quite new to writing code (about a few weeks) and I\'ve hit a wall while writing code for my website. I want to have a layout like this:

Answer 1

http://jsfiddle.net/kkobold/qMQL5/

#header {
    width: 100%;
    background-color: red;
    height: 30px;
}

#container {
    width: 300px;
    background-color: #ffcc33;
    margin: auto;
}
#first {
    width: 100px;
    float: left;
    height: 300px;
        background-color: blue;
}
#second {
    width: 200px;
    float: left;
    height: 300px;
    background-color: green;
}
#clear {
    clear: both;
}