For your information GST will be applicable to an entity only from the date which is mentioned in the GST Registration Certificate.
Also in your GST login only from the month of GST registered can be available to you for filing.
If there is no inward and outward in a particular month you have to file "Nil Return". Nil return means no such transactions made.
As usual you have to file all the prescribed applicable returns with Zero values.
If you didn't file you have to pay the penalty prescribed by the law.
Usually the fees will be varied on the basis of returns and Nil or Tax returns.