OK, so the other day my 2 biggest goldmakers on Gaia, the jigsaw puzzles and word bump, were both down. Guess I need to learn how to fish....
The first thing you should know is that there are 3 different places on Gaia where you can fish. Bass'ken Lake, The Port of Gambino, and the Durem Reclamation facility. Bass'ken Lake is the easiest, the Port is intermediate, and the Reclamation Facility is HARD!!!
Pull up some pier at Bass'ken Lake, and start casting. You should have your "basic fishing rod". You cast by clicking on the water. Watch the meter at the bottom, and click the mouse again when it reaches max. When you hook something, 2 red lines will appear. You'll need to move your mouse side to side, trying to keep the bobbin between those red lines, until you reel in whatever you've caught. This isn't nearly as easy as it sounds! Keep practicing, you'll start to get the hang of it.
Study your reeling, and take note where you seem to lose your fish the most. The other fishing rods offered will change where those red lines are. Take a look at this forum, http://www.gaiaonline.com/forum/viewtopic.php?t=7274895 for screenshots of the other available fishing rods, and how they change the red lines. I found that I lost the vast majority of my fish right at the shoreline, and switching to the "strength rod" really made a difference. Figure out where you lose your fish, and buy the rod that looks right for you.
Onto what you can catch...
If you are fishing because you need money DO NOT EXCHANGE YOUR FISH!!! Sell them for gold, but don't exchange them with old Man Logan. Guppies are worth 3 gold each, but if you exchange 10 of them, you get a bass, only worth 12 gold. You can sell any of the garbage you catch. Just like real life, the heavier/better what you caught, the more it will fight as you reel it in. This means that when you first start out, you're going to catch at least as much crap as fish, because garbage doesn't fight as much.
Fish can be sold for gold, or exchanged with Old Man Logan for better fish. As you collect more fish, you will be able to cash them in for items. I sent Foamy a set of whiskers made out of fish, ask him to show them to you, they're cute! I'm saving up for a hat....
Bait matters! You will definitely catch more, better fish with Grade A bait than with Grade F. However, when you're fishing with better bait, it can be harder to reel fish in, for obvious reasons.
You can move around. I don't know if it really makes a difference or not, but you can look at the little photograph of the lake on the left, and click where on the edge you want your avi to sit.
I've gotten to the point where I do pretty good in Bass'ken Lake, and can even recognize garbage by the slow reel before I actually bring it in, so I can release it. I now bring in almost entirely fish. When I go to the Port, I get about half and half. As for the Reclamation Facility, I haven't been able to reel a single thing in yet.
Good luck, hope I was able to help!