There are certain columns which are mandatory, how can i enforce that a
cell is not left blank?

With a change event:
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Column = 2 Then '2 for column B, CHANGE as necessary
If IsEmpty(Target) Then
MsgBox "Mandatory!"
Application.EnableEvents = False
Target.Select
Application.EnableEvents = True
End If
End If
End Sub

How can i force that data entered is numerical only?

select say column B,
Validation/Custom/Formula: =ISNUMBER(B1)

Regards,
Stefi