Hello, I'm Keith and happy to help you with your question. Your Form VAT4, Certificate of Registration for Value Added Tax, will tell you the quarterly dates upon which you make your return. This was sent to you to confirm your registration. It is imperative you keep this document; I know of a company where HMRC demanded a confirmation of the registration after some 10 years of trading!
Incidentally, the VAT on line system will not usually allow you to make returns for an incorrect period.