Like Jenn said, good oil ( Peanut is my preference) and hot 350 deg. PLUS cold fish and the ability to keep the oil @ 350 deg. when you add the cold fish.
A cast iron dutch oven does nicely. I like to dip my filets in buttermilk and usually use a commercial fish breading (some are better than others) to coat the fish after soaking.
After coating, refrigerate the coated filets while you get your oil hot.
Then, after your oil is hot (350 deg.) add 1 piece of fish at a time, taking some time before adding another piece. Plan on doing several smaller batches rather than trying to cook it all at once. It usually takes me 30-40 minutes to fry up enough for my family.
Fried fish, lemon, cole slaw, fries and a cold brew or 3 =
Bernzy