You need a male and a female Pokemon from the Field egg group. Although ratticate is field, they won't always breed. It does depend a bit. But make sure you got the things like the gender. Then just wait a while.
Basically, get any male Pokemon from that group I linked above. Make sure they rare high(er) leveled, and mix and match them a but until you get the egg. Egg moves and passing natures is different, but if you don't care about that then it doesn't matter.
hope I helped xD