How do I register for corporation tax?

You need to register for Corporation Tax with HMRC within three months of starting to trade, which you can do online using your company's Government Gateway account and Unique Taxpayer Reference (HMRC usually sends the UTR automatically shortly after Companies House incorporation). Registering late can result in a penalty, even if no tax is actually due yet. This is separate from registering for VAT or PAYE, which you'll need to do separately if they apply to your business.