The active volcano Piton de la Fournaise is one of the view active volcanos in the world considered to be "friendly" volcanos. This means that there is no sudden blast but rather ocassional "soft" erruptions which usually do little harm to the people of La Reunion. If you're lucky such erruptions can even be viewed from the second crater rim or during a helicopter flight over the volcano. From my point of view, Piton de la Fournaise is comparable to the active volcano on the Big Island of Hawaii.
The road to the volcano starts at the small village of Le Vingt-Septieme on road N3. There is also a visitor center but from my experience, if you don't speak french it's certainly a waste of time. After you have passed the view point Nez de Boeuf and the parking lot to the crater Commerson, a deep former erruption crater, you'll shortly reach the view point of the first rim. You overlook a huge caldera with the smoking volcano in the distance. What a sight!

Take the serpentines down to the floor of the first caldera until you reach a really nasty gravel road. From here on it's better to drive real slow because the road is extremely bumpy and it's probably not a good idea to crash your car there. After a shaky 20 minute drive you stop at the parking lot of the second caldera. The view to the volcano is really breathtaking! This is also the starting point for hikes to the top of the volcano. Unfortunately, when I was there the trail was closed due to ongoing erruptions within the volcanic crater so I can't tell anything about that hike.

Some general tips: try to start very early in the morning. Unless you have an extraordinarily beautiful day, the volcano is usually in the clouds by noon. Also keep in mind that the drive from Le Vingt-Septieme to the second caldera takes round about 1-2 hours (one way). If you wanna do the hike to the top of the volcano, best is probably to spend the night at the mountain hut Gite du Volcan close to the second rim.