Just Advertising ?

I'm in the UK and am registered as self-employed, so any earnings I make I simply submit to the Inland Revenue (HM Revenue and Customs) in my year end tax return (self-assessment).
From small to large website work, advertisement revenue - anything I "earn".

By law, you are supposed to declare and "possibly" pay tax and NI on any earnings you make, whether self-employed or not.
But there are various "complications" towards how much tax you pay and on what income. Your total earnings (and other things) determines what tax and NI you pay.

If you're self-employed on a temporary or part-time basis you must register with us as self-employed within three months of when you first start work. You'll have to complete a Self Assessment tax return and are responsible for paying your own tax and National Insurance contributions on the income you earn. Even if you don't think you'll earn enough to need to pay tax, you still need to complete a tax return.
Working for cash

You can accept cash payments for work you do, but it's illegal for you not to declare this on a tax return, and pay Income Tax and National Insurance contributions if these are due. This will depend on your overall taxable income in the tax year.
Quotes found here: http://www.hmrc.gov.uk/working/casual.htm

Here's a link that has loads of info from Inland Revenue themselves regarding self-employment, so it's 100% accurate and you can work by it:
http://www.hmrc.gov.uk/businesses/tmastarting-up-in-business.shtml

The self assessment for submitting your earnings is really simple. You just keep a record of what you have earned and anything paid out relating to your earnings, fill in a simple online form at the end of the year and IR calculate what you owe and when you have to pay it.
Being self-employed, you can actually choose to put anything you owe into the next tax year, so you can stagger your payments if you want, although putting some aside every time you earn is the best way, and makes it easier when you come to pay up.

So registering as self-employed and putting the website's earnings through as your own is the easiest way I find. There are benefits to registering the site as a business, but it also brings potential complications too.

Often, with a website, there is no need to register it as a business, unless for some reason you need it to be. The website from the link I gave above has info about all of this. And the few times I've phoned them they've been really friendly and very helpful.

wow thanks for your fast reply ! that was so helpful !