My IPhone 5s got stolen and they turn it off, I want to track them down but I don't know whats the IMEI no. I don't even have the box

You can't track it by IMEI. The only way to track it is using Find My iPhone.

Contact your wireless provider immediately. They will

be able to block the iPhone from being used.

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

If the device is turned off then you can't find it.

No way to track via IMEI.