Mvat registration & charging of mvat

if turnover exceeds500,000/- in the first bill raised itself the did i have to get MVAT Registration??

and also VAT is chargeable on what amount ?

on Invoice amount  or on  amount exceeding500,000/-

if on Invoice amount the that will be considered as inclusive of VAT or Exclusive ?

as the act only says :

a dealer shall not be liable to pay tax in respect of such sales as take place during the period commencing on the 1st day of April of the said respective year upto the time when his turnover of sales, as computed from the 1st day of April of the said respective year, does not exceed the relevant limit applicable to him ie. 500,000/-

Kindly help and suggest me ...