how can i know when is the first day i used my iphone?

There's no way to determine, exactly, the "first day" you used your phone. However, iPhone warranty/support starts the first day an iPhone is sold, since most users start using the phone that day, that's a prerry good indication of the day you may have first starting using the phone.

Plug the SN in here:

https://selfsolve.apple.com/agreementWarrantyDynamic.do

That will tell you the day warranty/support started.

If you ask nicely, your carrier may tell you the day the phone was activated on your account.

You're welcome.