Validation in in textbox using visual basic

I am about to do validation on the textbox using vb..can someone help me how to validate if i leave the text empty using vb language? N how to validate the character, phone number and email?

    Imports System.Data
Imports System.Data.SqlClient
Imports System.IO
Imports System.Web.UI.WebControls


Partial Class _Default
    Inherits System.Web.UI.Page
    'Dim con As New SqlConnection(ConfigurationManager.ConnectionStrings("dbconnection").ConnectionString)`


Dim con As New SqlConnection("Data Source=.\SQLEXPRESS;AttachDbFilename=C:\Users\user\Documents\Visual Studio 2010\feedbackScholarInn\App_Data\Database.mdf;Integrated Security=True;User Instance=True")

        Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
            Dim date1 As Date = Date.Now
            con.Open()
            Dim cmd As New SqlCommand("insert into complainer(fullname,no_Tel,email) values(@fullname,@no_Tel,@email)", con)
            cmd.Parameters.AddWithValue("@fullname", txtFullname.Text)
            cmd.Parameters.AddWithValue("@no_Tel", txtno_Tel.Text)
            cmd.Parameters.AddWithValue("@email", txtEmail.Text)
            cmd.ExecuteNonQuery()
            con.Close()

            con.Open()
            Dim cmd2 As New SqlCommand("insert into feedbackDetails(message,subject,status,typeofFeedback,date,roomNo) values(@message,@subject,@status,@typeofFeedback,@date,@roomNo)", con)
            cmd2.Parameters.AddWithValue("@message", txtMessage.Text)
            cmd2.Parameters.AddWithValue("@status", "New")
            cmd2.Parameters.AddWithValue("@subject", txtSubject.Text)
            cmd2.Parameters.AddWithValue("@typeofFeedback", dropDownFeedback.SelectedValue)
            cmd2.Parameters.AddWithValue("@date", date1)
            cmd2.Parameters.AddWithValue("@roomNo", txtRoom.Text)
            cmd2.ExecuteNonQuery()


            con.Close()


        End Sub
    End Class