While you can open Manage | Database | fields and manually reset the "next serial value" setting for an auto-entered serial number, you can also use Set Next Serial Value in a script to reset the serial number setting. A script can run every data from either a server schedule or each time you open the file that can check the date and when it's the first business day of a new year, resets this file number serial, back to zero.
Similar scripting can set the next value back one value so that you can change a new record's serial ID to a "B" format and then run such a script to avoid a gap in your numbering series if that is a concern for you. I definitely recommend that you specify a Unique Values validation on this field so that you can catch mistakes that would otherwise result in issuing the same file number for two different records in your database.
You can include leading zeroes as part of the serial number field's next value setting if the field is of type text instead of number.