Hello Mario
Assuming you're talking about a sole trader, in which case from the horses mouth:
Register as soon as you can after starting your business. At the latest, you should register by 5 October in your business’s second tax year.
Example
If you start working as a sole trader between April 2014 to March 2015, you should register before 5 October 2015.
www.gov.uk/set-up-sole-trader/register
In other words sooner the better!
