Netfile and Efile have different process in filing a first time filer's tax return.
For Netfile software, there is no special field to enter for a first time filer. Only tricky part is if tax payer has a middle name registered with Government, then enter the middle name in first name field on page 1 of T1 after the real first name (note, there is no middle name field in T1 return). When netfile process starts, at the stage of Netfile term and conditions, it will prompt a question asking whether tax payer is a first time filer, choose Yes then proceed.
For Efile software, there are radio buttons under name field on page 1 of T1. It's labeled "did last name change in tax year?" For first time filer, please choose Yes for this question. It only means CRA system will establish a new name record for this tax payer, so it will create a new record for this first time filer. Efiling process is just same as filing normal returns, there are no special question asked.