Adding:

a = Val(TextBox1.Text)
f = 1 (flag that tell the operation to be performed)
TextBox1.Text = ""
d = 0 (this variable is for point)

Subracting:

a = Val(TextBox1.Text)
f = 2
TextBox1.Text = ""
d = 0

Multiplication:

a = Val(TextBox1.Text)
f = 3
TextBox1.Text = ""
d = 0

Division:

a = Val(TextBox1.Text)
f = 4
TextBox1.Text = ""
d = 0
Equal:

If (f = 1) Then
TextBox1.Text = a + Val(TextBox1.Text)
ElseIf (f = 2) Then
TextBox1.Text = a - Val(TextBox1.Text)
ElseIf (f = 3) Then
TextBox1.Text = a * Val(TextBox1.Text)
ElseIf (f = 4) Then
TextBox1.Text = a / Val(TextBox1.Text)
End If

Displaying Text in the button Which is clicked:

textboxname.text+=buttonname.text
(e.g: textbox1.text+=button1.text)