I wrote a long and detailed tutorial on it http://procbits.com/2013/08/27/generating-a-bitcoin-address-with-javascript

In short, scroll all the way down to the summary to see this short version:

var randArr = new Uint8Array(32) //create a typed array of 32 bytes (256 bits)
window.crypto.getRandomValues(randArr) //populate array with cryptographically secure random numbers

//some Bitcoin and Crypto methods don't like Uint8Array for input. They expect regular JS arrays.
var privateKeyBytes = []
for (var i = 0; i < randArr.length; ++i)
  privateKeyBytes[i] = randArr[i]

var eckey = new Bitcoin.ECKey(privateKeyBytes)
eckey.compressed = true
var address = eckey.getBitcoinAddress().toString()
console.log(address)// 1FkKMsKNJqWSDvTvETqcCeHcUQQ64kSC6s

var privateKeyBytesCompressed = privateKeyBytes.slice(0) //clone array
privateKeyBytesCompressed.push(0x01)
var privateKeyWIFCompressed = new Bitcoin.Address(privateKeyBytesCompressed)
privateKeyWIFCompressed.version = 0x80
privateKeyWIFCompressed = privateKeyWIFCompressed.toString()

Open up the JavaScript console on the page (in your browser) and follow along. You can create your very own address right there in the browser. I wouldn't recommend using this newly created address to actually conduct commerce. Just use it as an academic exercise.

There are a number of sample programs here: https://bitcointalk.org/index.php?topic=1026.0

Also see the wiki for the step-by-step algorithm for generating a valid address.

There are plenty of very easy-to-use tools that can generate addresses for you, the exact process is detailed in the Bitcoin wiki.

Some examples of such tools are:

• BitAddress.org
• BrainWallet