"Ga" always denotes the subject, while "wa" denotes the topic which may be the subject, object, or something else entirely. That's how you can have sentences like "Suzuki-san wa nani ga ii desu ka?" (What would you like, Suzuki-san) or "Tokyo wa hito ga ooi" (in Tokyo there are many people). Or when ordering drinks you can say "watashi wa biiru desu," which means something like "a beer for me," not "I am beer."