I bought a female marble cow, but when I tried to milk her a "?

You need to wait 30 days after the day you purchased her so she can grow, then you need to breed her which will take about 30 days before you'll even be able to get milk...sucks doesn't it : ]

First you'll have to wait until she's mature. You can tell because she gets a mysterious growth spurt in the middle of the night. Then, you buy a bull or order a miracle potion and breed her.

Wait until the calf is born, and feed it any Mother's Milk your cow produces. About 5 days after that, it will produce B milk or higher, depending on how you treated your cow before the birth, and feed it that until it no longer accepts it. Then you give it Fodder whenever the container in the calf hutch is empty. Hope I helped!