Coming by train
There are high-speed trains to Paris from neighbouring countries. For instance, the TGV high-speed train runs direct from Munich to Paris several times per day in about 6 hours.
The trains to Saint-Malo leave from Gare Montparnasse, which you will likely not arrive in. Trains from the north arrive at Gare du Nord and from the east at the nearby Gare de l’Est (also written as Paris-Est). You can get to Gare Montparnasse by underground metro (you can plan the trip here on the RATP website) or taxi (30-45 minutes).
The train from Gare Montparnasse is direct to Saint-Malo, although it depends on the travel you purchased. Some trips will involve switching trains.
We highly recommend you reserve your train tickets (to Paris and to Saint-Malo) well in advance. The prices rise substantially for last-minute travel. You can purchase train tickets online and print them out at home, or collect at a machine in the train station in France.
Be sure to leave ample time for delays and inner-city travel between arriving in Paris and catching the Saint-Malo train.
Coming by plane
It is best to fly to Paris as most international flight land here. Although you may want to check for flights to Rennes, a city nearby Saint-Malo. For instance, there is a direct flight from Amsterdam to Rennes on KLM airways.
When flying to Paris you may choose either to Charles de Gaulle Airport (also known locally at Roissy Airport), or Orly Airport, a smaller airport nearby in the south.
Once you arrive in Paris, the process is very similar to arriving by train. Use the local transportation to get to Gare Montparnasse.
From Charles de Gaulle Airport, you may take the RER B (urban railway) to the center and switch to the metro. There is a good article with more details here. It will cost around 11 EUR.
From Orly, take the OrlyVal train to the RER station at Anthony or the OrlyBus further into the city to Denfert-Rochereau. From there take the metro to Gare Montparnasse.
In either case, taking public transportion from the airport can be difficult, especially if you are not used to the Paris transportation network. In that case, we recommend taking a taxi. The taxis in Paris are efficient, safe and because the city recently introduced a set-price for travel they also can no longer rip off tourists. The set prices are 50-55 EUR from Charles de Gaulle and 30-35 EUR from Orly. Details here.