Level 1-20:
Go to Al-Kharid with a net. Go south of the bank and net there. If you plan on training your cooking level as well, then after you get a set, go to the range north of the bank and cook them there.
Level 20-40:
Kill chickens (their feathers are needed to catch fish). You are going to need a lot of feathers (at least 656). Buy a fly fishing rod from the fishing store in Port Sarim or from players. Have both the rod and feathers in your inventory and fish in the river east of Barbarian Village. If you are training cooking still, then use the fires in Barbarian Village bar to cook on. If you plan to keep them, there's a bank north of the fishing spot.
Level 40-60:
Here's where you'll be making lots of money. Make sure you have lobster pot (from Port Sarim fishing shop) and 60 coins. Go east of the Port Sarim fishing shop and right-click a sailor and choose pay 30 coins. You will now be on Karamja, the only place to fish lobsters on f2p (free to play) worlds. If you are a member, Catherby is the best place to fish for lobsters. Sell the lobsters raw or cook them and use them.
Level 60+:
You can continue to fish lobsters, or you can switch to swordfish. Swordfish heal more and are worth more; however, you will also catch tuna, making it longer to fish a set of swordfish. If you're a member when you get level 76 fishing, then you can fish sharks, that are worth lots more.
