If you obtained an Employer Identification Number (EIN or TIN) from the IRS, you will need to file a business income tax return (Form 1120-S for a subchapter S corporation). Applying for an EIN will trigger the systems within the IRS to look for an income tax return filed for that number. Failure to file a tax return as required may result in notices and penalties from the IRS.
If you did not file for and receive an EIN, there is no need to file a business tax return.
Please note that a corporation is a legal entity that is formed under your state's laws. If you have not registered with the state, you are almost certainly NOT legally a corporation and should resolve this as soon as possible, either by registering with your state or by contacting the IRS to figure out how to revoke your EIN.
