You must tell HMRC that your company is active for Corporation Tax purposes within three months of starting business activity. This can be performed using the HMRC’s online registration service.
You must have a separate Government Gateway account (User ID and Password) for each company that you want to register with HMRC for Corporation Tax. This will allow HMRC to automatically enrol the company for CT Online once your registration application has been processed.
Follow this HMRC link.