asp.net client / server

How could I call a server-side function from the client side, which returned results and use that data from the client side?

client-side code:

html:

<input type="button" value="Search" onclick="codeAddress();"</input>

javascript:

function codeAddress() {
    var variable1 = "medicina"
   //llamo a la funcion GetData //
   //guardo variables cada campo que devuelve la funcion GetData
   //...sigo con el codigo
   }

server-side code:

Private Function GetData(variable1 As String) As DataTable
Dim conString As String =ConfigurationManager.ConnectionStrings("constr").ConnectionString
Dim cmd As New SqlCommand("select * from location where especialidad =@variable1)
        Using con As New SqlConnection(conString)
            Using sda As New SqlDataAdapter()
                cmd.Connection = con

                sda.SelectCommand = cmd
                Using dt As New DataTable()
                    sda.Fill(dt)
                    Return dt
                End Using
            End Using
        End Using
    End Function