When you get to the airport go to departures. There will be desks for each airline. Find yours. They will print your boarding passes and take your hold bags (if you have printed them at home and only have hand luggage skip this step)
follow signs for security.
here take a small plastic bag and put into it any liquids you want to keep in your hand luggage (rules on how big are changing so google) all others go in the bin
someone will scan your boarding pass and let you thriugh
you then have to put all bags, coats, bulky shoes and contents of your pockets into trays. The people there will tell you if laptops need fo be out of bags.
traus are x-rayed and you go through a human scanner
if the bag looks odd it will be taken to one side and looked at with you
if you set the human scanner off you will be patted down
tbks is routine and not an issue ( I was once patted down multiple times until we established it was my bra wire)
you will likely now be in a massive mall. Shop or eat as you will
dottef around will be massive boards with all the flights. That will tell you your gate number. As soon as it says boarding start walking. Depending on the airport it can be a bloody long way and boarding closes usually 45 mins to an hour before the time on your ticket.
once you get to the gate show your tickets and passport and they will put you on the plane
hope that helps. I remember my parents neighbours coming with us to Gatwick when I was a kid because my parents were so worried about what to do. Shout if you have any other questions