You can find the answer to this in Chapter 7, right near to the beginning. The short answer is that Napoleon decides that they will use the lawyer, Mr. Whymper, to spread lies around among the people. That will prevent the people from knowing what is going on on Animal Farm.
Mr. Whymper is not going to know he is lying. Instead, they are going to trick him. They'll fill bins with sand and put a bit of grain on top to make it look like the bin is heaping with grain, for example. Mr. Whymper will see that stuff and he will tell the other people.