VBA using forms and textboxes

I am a bit confused as to how to implement code for Text boxes. I am trying to set up a form which displays a text box, for which the user has the option to enter in the values (parameters) , which then are entered into another subprogram and runs that subprogram. For example, in the following code:

Option Explicit

Sub FillSampleTable()

'This will fill an x by y grid of numbers incrementing to use for testing purposes

Dim x As Double
Dim y As Double
Dim z As Double
Dim OffsetColumn As Integer
Dim offsetrows As Integer
Dim a As Long

Application.ScreenUpdating = False

a = 10
OffsetColumn = 1
offsetrows = 1
z = 0

For x = 1 To a

    For y = 1 To a

        z = z + 1
        Cells(x + offsetrows, y + OffsetColumn).Select
        Cells(x + offsetrows, y + OffsetColumn).Value = z

    Next y

Next x

Application.ScreenUpdating = True

End Sub

I would want to have a text box to prompt the user for the value of 'a', and once that value is loaded, then this subprogram is run. If no value is entered, then it would shoot an error msgbox and return to the entry screen. When I click the box I get:

 Private Sub GridSize_Change()
    'I renamed text box GridSize

    End Sub

but don't know what to do with this. MS Excel v 2016.