This depends on whether you are already signed up for existing HMRC services:
if you are not already signed up for a HMRC online service or you are registered for the Self Assessment Service (as an individual) or for VAT Online Services only, then you will need to register and enrol afresh and you'll need to get a separate user ID and password to the one you already have
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.