Most of that are subjective design decisions that depend on your ability and other requirements. However, I'll give you some initial impressions.
vjvjvjvj wrote:which format the webservice should return the information .
Look into ASP.Net Web Services. You basically create classes and the serialization is handled for you. Depending what you want to do with the data and some other factors (e.g., amount of data classes), you may or may not want to go that route.
Not unless you need to do that.
That seems ideal.
vjvjvjvj wrote:Should it be session variable or viewstate?
If you don't need to pass it back and forth each time or you mostly need it on the server side, then use the session to store the info. That will lead to less traffic.
vjvjvjvj wrote:should the data be in dataset format?
Use whatever object structure seems most natural to use.