how to enable my button if all the required textbox is not null

See if this helps.

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        For Each txt In New TextBox() {TextBox1, TextBox2, TextBox3} '// your TextBoxes.
            If txt.Text = "" Then Exit Sub '// skip remain code in Sub.
        Next
        Button2.Enabled = True '// enable if all TextBoxes have values.
    End Sub

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Button2.Enabled = False
    End Sub
commented: Great help for sure :) +14

so you have to click button1 first to validate the textbox?

For my posted code, you do.
If you need to have it trigger when typing in TextBoxes, add the code to the TextChanged event of the TextBoxes.

Private Sub _TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged, TextBox2.TextChanged, TextBox3.TextChanged ', TextBoxETC.TextChanged
        '// validate TextBoxes code here.
    End Sub