There are six international airports serving London, so make sure you go to the right one.
On your ticket are codes for the airport and airline you travel with.
E.g. LHR and UA165 means you fly from Heathrow Airport with United Airlines, flight number 165.
LHR = Heathrow
LGW = Gatwick
LTN = Luton
STN = Stansted
LCY = London City
SEN = Southend