For guides, check the stickies in this forum you just posted in. Since what is and isn't profitable changes all of the time, you'll be hard pressed to find any up-to-date fishing-for-profit guides. There are certainly some things that never change (Moat Carp will always sell quickly),
Fortunately, it's easy to find profitable targets with a little reading. You'll need these two pages: http://wiki.ffxiclopedia.org/wiki/Category:Fishing/All_Recipes http://www.ffxiah.com/
Check out the first list, find what sells on the AH for a good price or look at how much an npc will pay. You can use that information to find profitable fish.
Finally, you'll also want a fishing rod. I strongly recommend a Halcyon Rod
as soon as you can. It'll catch most low level fish and (imo) the best rod next to the legendary ones. If you're looking for a cheaper rod, I guess anything will work, but it'll probably break before too long.
A bit about skill- fishing skill means very little, the rod you use is actually much more important. This means that even with lv0 skill, you will be able to catch Gold Carp, a lv56 fish, with the right rod. That is not to say you can catch all lv50+ fish, just that when you look for something to catch, you do not need limit yourself to fish around your level. You should be able to catch most anything under lv50 with a Halcyon rod.
Also, try to pay attention to the date on threads. This one is from 2011 and given he only has 9 posts, I doubt he's even still active on this forum.