Fish is done when its internal temperature reaches 140 degrees, but most cuts of fish are too small to use an oven thermometer. Cook for 10 minutes per inch of thickness at the cut's thickest point. Depending on your oven and your tastes, you may want to make that a minute longer or shorter. The flesh should flake easily when pricked with a fork. If it won't, cook it more.
Fish continues cooking if left in a hot pan or over hot liquid - so be sure to remove it promptly.
Don't overcook it. Don't cook one with a shell that is already open. Don't bother with fussy sauces until you're more confident cooking the fish itself. Salsa makes a great topping, as does any pedestrian honey-mustard concoction. Lemon pepper is about all any whitefish needs.