Basics of Fishing:
What do I need to start fishing?
Well just like the game tells you when you go to start, you need a fishing rod and some bait, which you can buy at "The ole fishing house" in Bass'ken Lake. Then you just click on the lake you want to fish at (I recommend Bass'ken Lake for those who are beginners) then click on the bait you want to use (I also recommend using F bait if you are starting out). Don't panic if you don't see your rod on the first screens, you get to pick the rod you want to use once you are in the game (if you have more than one rod in your inventory).
Basic rods are the norm for all new players.
For more info on rods and baits check out this thread. (First two posts)
http://www.gaiaonline.com/forum/viewtopic.php?t=4852325
(note: Only the regular strength rod lacks the ability to reach water's deep enough to hook a large fish, the best regular rod for the job is probably the distance rod)
Something is terribly wrong and my game won't start!
This guy has some good info on glitches that prevent you from starting a game. (3rd post down)
http://www.gaiaonline.com/forum/viewtopic.php?t=4858935
Also check the announcements to see if an error is occurring that Gaia is trying to fix.
Where do I find each fish?
Here's that post I told you had great fish info. (First post, third quote)
http://www.gaiaonline.com/forum/viewtopic.php?t=4594036
For the shorter, less detailed version go to this thread. (Third post down)
http://www.gaiaonline.com/forum/viewtopic.php?t=4858935
How does bait grade affect my catches?
The better your grade of bait the more likely you are to catch larger fish, and the less trash you will have to encounter during your game. This doesn't mean go out and buy A bait first thing, you may have seen posts asking about "Bait Degredation". Simply put it means when you wait twelve real time hours inbetween your fishing games you can buy lower grade bait that will act like A bait. The specific patern that you should buy to get A bait results are F-F-D-D-D then switch to A bait, or wait twelve real time hours to go again. More info here: (first post)
http://www.gaiaonline.com/forum/viewtopic.php?t=4858935
How do I find a rare fish?
You don't they find you. Rare fish are a random Gaia event like pick boxes, or pink links. They can show up at any distance, with any rod, with any bait (except for degraded F bait apperantly). For more info click here. (Second post)
http://www.gaiaonline.com/forum/viewtopic.php?t=4858935
How do the fish move/ How do I catch a fish?
Here's the good info post again, the second post shows how fish move; and further down is information on the speed/determination of each type of fish.
http://www.gaiaonline.com/forum/viewtopic.php?t=4594036
To catch a fish you must move your mouse around the screen in the opposite direction to stop a fish from moving before it goes out the red lines. I have also taken the liberty of making a diagram that shows how far you have to move your mouse on the screen to stop various fish.

In each lake I will explain how you can apply this information to filter out unwanted materials and get a good haul.
Now that I have these fish what do I do with them?
There are a number of things you can do with them. You can collect them in order to make a nifty fish hat (and now they have trash clothes too), which can be done at the "Fish Exchange" at "The ole fishing house".
Info on hats:
http://www.gaiaonline.com/forum/viewtopic.php?t=4852325
Or you can sell them off. While you can sell them at the fishing house you won't make as much money off of them as you would if you sold them through the market (link to the fish market where you can buy/sell/trade your fishies
http://www.gaiaonline.com/forum/viewforum.php?f=125). The only fish I would recommend selling at the fish house are the small fish of each lake (guppies, seedkins, & pebbos) since there is not much demand for them any more.