You were at least honest and upfront saying your a student so I'll take it this is a homework assignment.
It doesn't matter what version of the .NET framework but since you are using VS2010 it might as well be 4.0.
A webservice itself doesn't retain data. The data needs to be persisted somewhere, usually a database.
Remember your professor is probably monitoring this, and other sites, to see who does the work own their own and who asks for others to do it for them.