Reims is one of the most accessible cities from Paris. The TGV from Paris Gare de l'Est takes 45 minutes and trains run frequently throughout the day — it is genuinely easier to reach by train than by car given the parking situation in the city centre. The station is about fifteen minutes' walk from the Cathedral.
There is no international airport at Reims; most visitors fly into Paris Charles de Gaulle or Orly and take the train from there. CDG has a direct TGV link that can put you in Reims in around an hour. By car, Reims is about 150 kilometres from Paris via the A4 motorway, around 90 minutes depending on traffic.