I think it’s probably fitting that the Squid should handle the seafood fundamentals, no?

I think that it is common knowledge that the freshest fish is best and that also means a fresh mild smell. If it smells acutely fishy, stay away.
In buying whole fish, look for clear eyes and bright red gills. In its butchered form, firm skin and firm, bright, and mostly uniformly colored flesh is what you want.
You should also try to know a little about where fish can come from. Rather than get into it I’ll leave you to the research. There’s some pretty shady places to get your fish (ponds of nuclear waste, anyone?). Definitely do not attempt to make your own sushi without plenty of research.

Some points on preparation:

Use unserrated knives to cut your fish unless you need fish confetti and crappy edges.
Do not attempt to peel or skin your fish fillet without a fish knife. You will waste and destroy. If you are grilling it, leave the skin on.
Keep fish cold and covered as long as possible until you’re ready to cook it. It won’t go bad but it will taste better at medium since the flesh won’t cook as fast.
Do not marinate fish too long in citrus based marinades- they’ll cook, ceviche style.

People are rightfully picky about their fish; there are so many variances. Pacific salmon (think sockeye) is very firm and flaky- it benefits from cooking over lower heat, barbequing, or baking with a very wet marinade. Atlantic is definitely better for frying or grilling. Tilapia has a strong taste (as a lot of fish do) when you fry it skin on, but very mild when used skinless. Milkfish has about 4000 bones but tastes great stuffed and grilled. Pike has a super overpowering skin and should always be skinned first. Try it all and decide what you like.

Some final pointers: do not overcook your fish. Like everything else, it continues to cook while resting and tastes dry and sad when overcooked. If you’re missing our consistent message I’ll say it again- well-done is poorly done.
And last, if using fish to make a stock, stay away from fatty fish like salmon.

I think that’s it! I’ll update as best as I can, if there’s anything additional required or as questions come up.