Keep it short, keep it simple. To find your site, people need to be able to remember your address and easily type it into their browser. Aim to register the shortest name that your visitors will associate with your website.
But what should I register? .co.uk? .com? .net?
It is a good idea to register several similar domain names, particularly if you own a trademark for a certain name. If possible, register the .co.uk, .uk, .com, .eu and .net. These are the most popular domain names and by registering all of them you can help to prevent cybersquatters and potential lost traffic.