I- The start

Ok well theres one basic concept. Patience. If you dont have that stop reading. But anyway if your fresh off tutorial island, or just never got to fishing, its problubly new to you. The first thing you need is: A bronze axe(Or any of the above) a tinderbox, and a small fishing net.
Once you have this head to the fishing tutor in Lumbridge. After doing that get a few inventorys of shrimp, cooking them each time. Bring them to a general store and sell them. Doing this should get you maybe 300 GP head on down to Port Sarim and go to the fishing shop, sell your net and get a brand new Fishing rod, plus bait. If you have some time, also get a fly rod with feathers a harpoon and if your confident, a lobster pot. Now head to Karamja via boat and go to the dock and face the island, go right from there, there should be a dock with a bunch of level 50's using lobster pots and harpoons, go to a fishing place that says "Bait" fish there untill you get to level 10 fishing, cook up your sardines and ta da, you can cook herring, fish herring, cook sardines, and fish them too!
First step completed