How can i call a server method from a client event?

if you go the atlas section of this site:

http://www.asp.net/default.aspx?tabindex=5&tabid=47

you will see plenty of introductory help on calling to the server using atlas ajax.

I think this is what you are looking for:

http://atlas.asp.net/docs/atlas/doc/services/exposing.aspx#webpage