Try fishing for herring
To start fishing I suggest fishing shrimp at Granado Espada vis and banking them. Once your fishing level is 5 you may want to start fishing sardines, but you are more than welcome to continue with shrimps. You can also try fishing for herring at level 10, but it is quicker and cheaper to continue fishing shrimps until level 20. At level 15 fishing you will be able to catch anchovies as well as shrimps. This means even more fishing XP. Fish anything that I have suggested above until you reach level 20 fishing.