You will have your net as soon as you are off Tutorial Island. The best thing to do is to find a netting place and begin fishing. I strong recommend you that the suitable place to fish is at Draynor, which is west of the Lumbridge Castle.
Once you are fishing at level 40, you will be able to fish for lobsters. To save time, you can buy runescape accounts directly. They are great seafood for money making. Sell them anywhere for 50-100gp each. When you are getting ready for Swordfish, you must notice that Swordfish are not going to be fast paced at all, and they will take more time to catch. This is great place for the HP and great GP as well.
For non-members, you have to go to Karamja to fish for Lobby's and Swordies. Obviously, this is a pains taking work and costs quite a lot to go back and fourth banking it. So, it is better to be a member at this point.
So the best thing to do is to pay 5 USD a month and become a member. Remember that farmer100 has awesome member runescape accounts for sale.