You use Ga when the object you're talking about relates to you. For example:
Chiizu ga oishii desu.
Relating to me, cheese is good
(I think cheese is good)
Wa marks the subject, so if you used wa:
Chiizu wa oishii desu.
Cheese is good.
Desu just means (is). You use Desu when you're talking about adjetives (sp?). Deshita means was, Dewa arimasen means isn't, and dewa arimases deshita means was not. Imasu is for verbs. Imasu means is, Imasen means isn't, Imashita means did, or was, and uh...I don't know the last one. >< Google Tae Kim's Japanese Guide to learn all about particles and such.
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