If you have a booking reference, which should have been emailed to you at the time of booking, follow this link:
https://www.egyptair.com/English/Pages/CheckMyTrip.aspx

Enter the booking reference and the name of the lead passenger and it should bring up your booking details.

Once there, you have the options to select seats and request special meals, though to be honest the flight from SSH - CAI is likely to be so short that you'll probably only be served a drink (if you're flying Business Class you'll get a snack).

Once any choices are made, save your changes and job's a good un, as they say.

Don't be surprised if the seat numbering looks odd. Neither MS nor MS Express start their numbering from row 1for some unknown reason