Thank you very much for the information. I added a few more things on the same subject. I included the doubts in parentheses.
1)Go (not pass) through the metal detector (?) .Go through passport control where an official checks your passport.
2) You have your ticket checked. (have your ticket checked?)
3) Go into the departure lounge where you can buy things in the duty-free.
Here you can wait before going to your gate number and boarding our plane.
4) As you have never travelled by plane, I'll give (in this letter I'm going to give?) you some instructions (information is possible?) on what you should do before boarding a plane (on what to do before boarding a plane).
5) If you have a hand luggage (a piece of hand luggage/any hand luggage?), you have to (should?) put it on a counter before walking through (before you walk is possible?) the metal detector.
6) Then you "ought to, you'd better" go into the departures lounge. Then you shoud go to your gate number, where you wait before boarding the plane.
7) Finally, board the plane and find your seat.I hope you'll have a good holiday. See you. Be careful and have a good trip.