How do I sign up for the CIS Online service?
This depends on whether you are already signed up for existing HMRC services:
If you read 'What details do I need to sign up for the CIS Online service?', this will tell you what you need to know before registering and/or enrolling.
Once you've signed up for the online service you'll need to activate it - look at 'How do I activate my online services?' if you need more help.