What kind of items I need to prepare for fishing in the sea?

The question is much too broad -- are you fishing off a pier, casting from shore, or going out on a boat, trolling, bottom fishing, or surface fishing? I own over 20 rods and reels and half a dozen tackle boxes of gear trying to cover all of that, and if I moved to the other coast I'd probably need a bunch more for the different style of fishing there.

Beyond tackle -- dress for the worst weather likely, dress in layers so if it's nice you can take some off and you don't melt. Sunglasses, no matter how sunny it is, and a hat. Sunscreen if it's going to be sunny. A rag or small towel for wiping or drying your hands. A camera.

A fishing license or permit if required in your part of the world. Seasick pills if you're susceptible and you're going on a boat.

Diagonal clippers for cutting the line and trimming knots and unhooking fish. A knife only if you're going to be cutting bait or cleaning fish.

And mainly, a good attitude. Sometimes you get fish, sometimes you don't, so don't let a slow day out there get you down.