Fishing is fairly simple - you cast a lure, then wait until the fish nibbles your lure as it creates a little ' plink ' sound. The idea is to wait until you hear that exact sound, and then to press space, to then see if you've gotten a lucky catch! :') If you press your spacebar too early or too late, you're not going to catch your fish.
Also, different fish like different lure. If the fish that you attract with your lure is a different school from the lure you cast, there's a more than likely chance the fish will swim away / you'll loose the fish. For example: you cast a life lure, but a death fish gets attracted to it. More than likely, if you do attempt to catch the fish you will not be able to get it, even if your timing was perfect.
There's tons of guides and tutorials and whatnot around, if you need help just do a quick search on youtube or google, or check the fishing tome in the commons :)) I'm not too bright at explaining things all too well, but I hope these bits & pieces of info could help at least a slight bit! xoxo
Some will always get away...using the right bait and having good timing are the most important things.
I think timing is the most important part of the equation, even more important than correct bait. I find its easier to listen for the popping noise rather than trying to watch the bait drop into the water, but maybe that's just my best method. Sometimes once I know a fish is attracted to the bait, I just close my eyes and listen for the pop...it works well for anything but sentinel fish...for those I just activate as soon as they zoom in and get lucky half the time.
Make sure your lure is the same school and rank as the fish you're trying to catch (e.g. for Lifeacuda use a Rank 1 Life lure). Don't click the space bar until the fish pulls the lure under (there will be a "plunk" sound, different from when the fish just nibbles the lure - keep your sound on). Some fish will nibble 3, 4, or even 5 times before pulling it under. Rare and Epic fish get away more often than Common fish. Other than that, it's like fishing IRL - just be patient!