Start off by teleporting to the fishing skill teleport which can be found in ur quest book in "skilling teleports"
Next you will be fishing by a small fishing net from level's 1-35
Next you will be fishing with a harpoon from level's 35-90
Next you will be fishing rocktail. To fish rocktail you will need to buy a fishing rod and about 200 Living minerals to get from 90-99 fishing. To find rocktail you can simply teleport to ::home and there should be a bubbling water on the ground next to the ancient altar and lunar altar. (fishing raw rocktail is amazing money, best money maker ingame!)
Goodluck.