Small rooms can be booked by editing this google doc file. Instructions are in the file.
For large rooms, i.e. rooms used for classes, typically at the third floor, the procedure requires an ENS account (to get an ENS account see the proper guide). To check the availability of the room, one should log in the intranet of ENS, then "Comment faire pour" -> "Reserver un salle", so here. If the page doesn't allow you to reserve, you can find the room responsible here and ask him.