Web Service authentification server site

[C# Programming]

Hello

I'm a newbie with WebService creation
For now, everyhing works fine, my Service run on a server and my Client can use it

Now I need to add autentification to the service ...
How can I do that simply on the server site ?

It does'nt need to be very secure, I just want to understand how to do it

I have an example of another service that I'm using as a client

I just have to assign a NetworkCredential to the service's client instance to be able to use it

How can I achieve that on the server side http://www.barakasoft.com/script/Forums/Images/smiley_confused.gif

Thanks a lot for any help