Well firstly you can only have a site that is available and nobody else has got.
You can use a site like this below to search what domain names are still available:
http://netcetera.co.uk/Products/domainnames/
Secondly you need to decide if you are going to have a uk based site such as co.uk or org.uk (which are cheaper) or .com or .org (which are more expensive)
And thirdly, maybe you need a quirky name that people will remember.
So www.cookerysite.com is a bit boring but
www.time4dinner.co.uk for example is not (which as it happens is not available).
Note that if you did register ww.time4dinner.co.uk then there is nothing stopping anyone else registering www.time4dinner.com and www.time4dinner.org and so on so unless your register ALL thse names your site may get lost.
So you need a quirky name, an original name, and an easy to remember name.