Koi forums Koiphen and Koi-bito are two of the best. But like waterbug stated, do your homework and give every detail you possibly can. Most want give you much help until the know size of pond, 3 of fish, size of fish, water parameters including but not limilted to ammonia, nitrite, nitrate, PH, General hardness, Carbonate Hardness, o2 level, and many will want to know metal content and phosphorus as well.
Good book I would recommend, Koi Kichi by Paul Waddington,
Nishikigoi Mondo [U.S.A. & Canada]
LWS (Living Water Solutions)
Email: (e-mail address removed)
and Erik Johnson's Koi health and disease. (amazon.com)
I will tell you you can read 5 books and get 5 completely different answers to a single question, but reading and having the material to find more than one way to do something will give you options. I personally have been kepping koi for about 6 years, 2 a lot more serious than the previous four. I have multiple books, and I read forums on average of one to four hours a day DAILY! and I'm by far still a novice. Koi can be one of the most demanding fish, but with proper care and good education they can be some of the longest living pet fish and can give many many hours and years of enjoyment. But it comes at a cost, no matter what they are not cheap fish to keep, you eaither spend money and time learning and applying or you spend money treating and retreating and spending money on more equipment to treat. or you spend more money replacing fish. IMO, a good test kit, a couple of good books and keeping a journal of what your water is doing will be an expense but one that will pay ofr itself many times over by being preventive instead of treating.