Saint-Denis is the capital of Reunion Island, which is a French overseas territory.
St Denis is quite unknown apart from the the French, so the only way to arrive would be from Paris. The way would be to arrive by plane (Air France) to Gillot Airport. This Airport is very far east from St Denis, but is near the north coast of Reunion island.
Travel around St Denis might be hard due to a handful of roads that were destroyed during 2013 Cyclone Dumille, however most of St Denis has recovered from the the Cyclone and roads are slowly being restored to their former excellence.
Most locals travel by car, motorbike or bus. The best way for a tourist is by bus though it'll probably cost 10 euros or more. Buses in St Denis are found in the City Center which is West of the Airport.