ExecuteScalar will return a single value....that's what you want.

untested, but this is roughly what you want:

Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click

myconnection.Open()

'define the command

Dim cmd As New SqlCommand

Dim i as Integer

cmd.Connection = myconnection

cmd.CommandText = "SELECT COUNT(*) FROM petrol_costs_table"

i = cmd.ExecuteScalar()

myconnection.Close()

MsgBox("Success...." & i, MsgBoxStyle.Information, "Success")

End Sub