There's a huge difference between them.
ga -- Marks subjects of sentences and clauses.
John ga kita -- John came (John is here).
Sakana ga oishii -- Fish is tasty.
o -- Marks direct objects of sentences and clauses.
Sakana o taberu -- I eat fish.
Biiru o nomu -- I drink beer.
Hae o koroshita -- I killed a fly.
Notice that in the
o example sentences that there aren't subjects. In Japanese, known information is usually omitted, so it's safe to assume that the subject of these sentences is the first person.
wa -- Marks topics of sentences. This is a bit difficult to grasp for native English speakers, because we don't mark topics like this -- we just understand that they're topics, whether we know it or not. Most times it's used like a subject marker, though. I can actually use the above sentences with
wa, too, like this:
John wa kita -- John came.
Sakana wa oishii -- Fish is tasty.
Sakana wa taberu -- I eat fish (but not meat).
Biiru wa nomu -- I drink beer (but not wine).
Hae wa koroshita -- I killed the fly (that we saw earlier; that we were talking about...).
I think that maybe
this page will be useful in your understanding of
wa, but it may take some time to really grasp it. Also, for basic grammar, I suggest browsing around that site (the link is in my sig).