As far as I know there are three different lounges, consisting of
150cc Lounge,
200cc Lounge, and
Battle Lounge. All three of them have their own set of rules and are divided into ranks. Depending on the lounge, ranks can access certain tiers in order to play events in the mode that the lounge is for. Your rank is determined by your MMR, and when you first join you'll have a starting placement in whatever rank you're given. All three of them are pretty active, but the 150cc Lounge has the most activity by far, so I suggest playing there to start.