Hi Akana! If you're planning on staying in San José you don't really need to get a car, there's buses everywhere and most of them go to/from downtown to any other place, there's also Uber and the "red" taxis, though you gotta be careful cause some of them can take some advantage of foreigners.
If you're going to the beaches/mountains or so you can take a bus to some of them, but it's way better to have a car then since its kind of a long ride and some of them have to be bought ahead of time.
Hope you enjoy Costa Rica!