If you are getting into breedables to make money, I would stay away. If you are doing them just for fun with the understanding that you will probably put a lot more money into them then you will get out of them, then go for it and have fun!!
As stated already there are tons of breedables already out and some in open beta.I am just going to rattle off a few of the breedables. There are dwarfins(yes dwarves, they don't "mate" but "create" other dwarves from rocks).Mossms ( which are fun but take up alot of land space because you need to rez their "environments" for them to work on, like a farm, excersize equipment, hot tubs, etc), WK Big Cats (Tigers, cheetahs, Jaguars). Gempandas(still in open beta) Imaginimals( formerly wildwood cats, these are also in open beta). Fennux, Meeroos, Amaretto Horses,Battle Beasts (Dragons, I believe).
There are also Krafties, which do not breed, but you"capture" other Kraftie creatures and boost up the stats of your Kraftie. They are 100% free, no food, needed, but you can buy extras like energy,etc. but it is not needed.
As Far as land goes,if you get into breedables "bigtime" there areland owners that rent land just for those who do breedables, so neighbors don't complain about the so called lag.
If you do decide to try out any breedables, join the main group for the particular breedable you are doing and read all the info on their website. Most give a comprehensive online manual.Also, it wouldn't hurt to make a post in the group chat introducing yourself and saying that you are new to breedables.Many established members are always willing to help out.
As far as links go, I will try to paste few that may be helpful, but as far as I know there is not one site to exlain everything about breedables , because each breedable has their own way of doing things.
The first link is just a general site about most of the breedables available in SL
http://sygnus.org/secondlifepets/
http://krafties.com/
http://www.imaginimals.net/news.html
http://mossms.com/
http://www.dwarfins.com/
http://gempandas.com/
Hope this helps, if you have any questions youcan always Im me in world and I will do my best to help. Good luck and have fun!!