Fishing is a very useful skill, not only to make money, but for yourself to use. It can also help you with the above method, Dragon & Giant Slaying. You need a minimum of 50 Fishing until you can start making a big profit from this method.

>From level 1-20, fish 447 Shrimps from Al Kharid.
>From level 20-50, fish 1,938 Trout from the Barbarian Village or Shilo Village.
>From level 50-80, fish 20,942 Lobsters from Catherby or the Fishing Guild.

Lobsters sell for around 100-200gp each. At level 80, fish for Sharks and sell them for about 700-1,000gp each.