Saint Denis is a suburb (banlieue) just north of Paris.
The city grew around the Basilica of Saint Denis, where his relics are currently stored.
To get to Saint-Denis, take the Métro 13 (light blue) and head north: the terminal is St. Denis Université, but get off one stop before at Basilique de Saint-Denis. No additional fare is required.
Although Saint-Denis is located in the suburb of Seine Saint-Denis, the city itself isn't as bad as other towns within the same départament (county). But as in any place, common sense of your surroundings is to be held