If you are going to be flying in, you would probably fly either to Heathrow, Gatwick, or Stansted airports.  Heathrow Airport is about 66 miles from Maidstone.  Gatwick  Airport is about 41 miles from Maidstone.  Stansted Airport is about 60 miles from Maidstone. London itself is about 40 miles from Maidstone, so it's very easy to get to the town if you are staying in the city.  Once you get to the airport, you can get to Maidstone from a number of different sources.

Train - you will have to travel via London.  Trains with Southern or South Eastern railways leave London from Victoria and London Bridge stations all the time, and you will have no problem getting a ticket.  There is no need to book in advance.  As these are commuter lines you may find them crowded in the rush hours.

Rent a car - from the airport if you want to do the driving yourself.  There are several routes you can take to get to Maidstone.

Coach to / from Heathrow - There are frequent coach journeys made to and from Maidstone to Heathrow via London Victoria coach station with just one stop involved.

Coach to / from Gatwick - Regular coaches now run between Gatwick North and South terminals and Chatham, Gillingham, Hempstead Valley shopping centre, Maidstone East train station, Allington, Borough Green, Sevenoaks and Godstone.  There is no longer any need to go anywhere near London if you're flying to or from Gatwick.  You can book a single or open return ticket online and check timetables at airportbycoach.com.  If you're flying into Gatwick South terminal just follow the sign for buses/coaches and wait at bus stop 19.  Easy and much cheaper than paying to park at the airport.