Just my $0.02 worth, but here goes.
If this is going to be your tradeskill, your first major purchase should be a composite rod. It runs about 8-9k at my AH, but it's worth it. Get a carbon rod too if you can afford it, but you can get by just fine on a composite.
Target fish that are 10 to 15 points above your skill level. Moat carp are fine to start with, but they skill out at 11. Once you hit 8 or so buy a Fly Lure and head to East Ron for Cheval Salmon and Shining Trout.
Once you hit Fishing 15, head to West Ron or Port San with a Sinking Minnow Lure and go for Giant Catfish. These catfish will take you into your mid 20s, and sell to any vendor for about 200g each. If you're in Port San, there's a vendor who will buy them in the Rusty Anchor Inn right there in the port.
At this point, you should start weaning off the moat carp as your source of income. They are small-time, you can do better.
Once you're in your mid 20's, take your composite rod and sinking minnow and head over to windhurst. If you take the ferry out of selbina, go ahead and grab a few extra sinking minnow lures from the shop on the boat. Head out to the cliffs in West Sara. You'll be after Orge Eels and if you get lucky a few Gold Lobsters. This should drag you up into your early 30s, which is where I am now.
Here, I've been switching out between the cliffs in W Sara and the lake in E Sara. I get more skill raises at the cliffs, but if I need fast cash I pull monkie-onkies out of the lake. Btw, if you use W Sara, you can zone right into Port Windhurst where the fishers guild is located. If you use E Sara, you got to run all the way across windy woods and most of the way across windy port to get to th fisher guildshop.
By this point your Moat Carp days should be a distant memory, they're a waste of time from here on out.
Moat Carp=3-4k/stack, or 250-333/fish. Lobster=400+/fish, Monkie-onkies=800+/fish. Oh, and in Windhurst, Giant catfish sell for 300/fish instead of the 200/fish I got in Sandoria.
I'm not sure what I'll do after this. I'll probably skill-up on the cliff until the I get above 35, then Monkies until 45. I'll let you know though.
Here's a couple tools that'll make life easier:
BlackenedForge has an excellent guide listing fish by location and lure. http://www.blackenedforge.com/fishing/
The Osakana Database. Lists fish, bait, location, and if your rod will break. http://www.fishermanrods.net/database_e.cfm
The fishing database at ffvault.ign.com (which for some reason is down this morning). Gives you a list of fish, their skill level, and approx sale price to the Fisher guildshops.