The things sushi chefs look for in their fish--

The eyes should be clear.

The gills should be bright red, no slime.

The scales should not come off easily.

When you press the fish, the skin should "bounce back" as if one pressed one's own forearm or cheeks.

It should not have any fishy smell.

And yes, sashimi-grade fish is frozen in the US, Canada, and Europe, mainly due to health regulations. The only fish that doesn't need to be frozen is tuna, and that's only because it isn't prone to parasites.

The U.S. Food and Drug Administration (FDA) recommends freezing at −35°C (−31°F) for 15 hours, or at −20°C (−4°F) for 7 days.[5]

I have learned alot from this guys videos how to prep fish. He is doing it really slowly for the video :) http://www.youtube.com/watch?v=SVFMjmb_Cf0&feature=channel_page

Here's the skinny on fish parasites.

The anecdotal evidence I've always used is don't worry about it in saltwater fish you've caught yourself. Nematodes are what you might worry about, but you probably eat things already every day that are more likely to make you sick. Nematodes do not find us to be a suitable host. If you're talking freshwater fish, though-- some eels, and trout come to mind as suitable sashimi freshwater fish-- you can theoretically get a tapeworm. So definitely freeze them.

As for prep, just cut your hunks such that you end up with the grain going across the short side of the piece. There are, of course, many rules and techniques beyond that. But that's the big issue to ensure you can easily bite through a larger hunk.

There is no such thing as "sushi-grade" fish. I think the only regulation is all fish must be delivered to the restaurants and markets already frozen. So I'm thinking freezing your catch ASAP will make the most fresh sushi.

I've also heard that the fish should be frozen, but not kept in the freezer too long. I assume your fish is pretty fresh since you caught it. I'd probably just cut it up and eat it, to be honest, but I'm pretty adventurous with food :) You could also maybe try doing a light ceviche so that it cooks slightly in an acid... Another preparation would be to cure it (like how lox is made).