Can I delete all stuff on my macbook air (to make it clean as if I just bought it) after I did backup with time machine? How can I do that?

Hi,

I need to rollback my macbook air to the state as if it was just bought. I am doing backup to external hard drive using Time machine now,

1. How will I know when it is complete?
2. What should I do next? how to delete all stuff from mac?
3. In a while, I will need to restore it back from the external drive. How do I it?
4. 

Thank you very much in advance. I'd really appreciate your fast reply.

All the best!