Realistically the only way to explain how to do sub netting is to answer the questions really. But explain how you got the answer. I will start with a few things that you need to know.
A IP address can't end with 0 or 255. So this means for each class C subnet you can have a total of 254 IP addresses.
Class A subnet is as follows 255.0.0.0
Class B subnet is as follows 255.255.0.0
Class C subnet is as follows 255.255.255.0
Each part of the ip address is broken down into Binary.
So Binary for a Class C Subnet would be
But I have just found something that will help explain.http://www.learntosubnet.com/Subnet_Section_helper.htm
It is a very hard thing to get your head arround. Basically either learn and remember how many subnets go with which subnet mask and where the IP addresses run from each subnet or get really good at binary.