I'm not 100% sure, so please someone correct mine as well if it's not correct or needs fixing.
First off, generally, "ga"(が) shows that the word before it is the subject. "o"(を) shows that the word before it is the object.
In this particular sentences, here are the meanings:
すしを食べます。 sushi o tabemasu. = I eat sushi. (the word "I" is implied, of course.)
すしが食べます。 sushi ga tabemasu. = The sushi eats it. (and due to the conversation, both you and the listener know exactly what the sushi is eating... even though that's not possible haha)
the verb "taberu" (食べる) takes "o" (を).
so:
BLAHBLAHを食べる BLAHBLAH o taberu = to eat BLAHBLAH
blahblahが/は(BLAHBLAHを)食べる blahblah ga/wa (BLAHBLAH o) taberu = blahblah eats (BLAHBLAH)
BTW: From what I hear, the verb "taberu" (食べる)cannot go without an object. Meaning, you can't just say "i eat". It has to have an object like "I eat SUSHI", or the object in which youre eating must be known to both you and the person with whom you a speaking.
Sorry for the bad explination. Hm...
In all, the thing you (or whoever) are eating will take "o" (を). The person/thing that is doing the eating will take "ga"(が) (or "wa"(は)).
Hope this helps ^^;;
がんばってくださいね!
-Tommy