Although the Ponta Negra beach in Manaus is better known, the Praia da Lua beach is probably a better beach overall. In both cases, the beaches are best (and biggest) when the river level drops, roughly August through December.
Praia da Lua lies beyond Ponta Negra, upstream on the Rio Negro, so you can make a day of visiting both if you like. To reach Praia da Lua beach, you will need to catch a boat from the marina just past Ponta Negra. You can get to the marina by bus, and then the boat trip to Praia da Lua is about 10 minutes one-way.
Keep in mind that there are places to get a bite to eat or something to drink around the beach at Praia da Lua. It can also be considered more of a short getaway from the busier beach scene of Ponta Negra beach.