first, of course, you need to fish somehting that is for level 1:
crayfish(lvl 1-15)
shrimp/anchovy(lvl 15-25)
trout(lvl 25-30-55)*from level 30 you start fishing salmon too)
lobster(lvl 55-99)(or 65 for sworfish) * for money in f2p*(55-82 on members)
swordfish/tuna(65-99)*for fast leveling on f2p*(65-82 on members)
and for members here's what to do after swordfish:
shark(82-88)
cavefish(88-93)
rocktail(93-99)
you can also get a profit of 70m+ after you get 99 if you start selling on lobster and swordfish