I will be flying to Tokyo 18th August reaching there early in the morning and I wish to connect a train to Sapporo as it will save me some money for other expenses.Consider flying. If you buy the domestic flight in combination with your international flight, it might be just as cheap as the train ride, if not cheaper. And much faster!
Will I be able to get the train nearby Narita airport..
You have to catch a train into Tokyo first (about one hour) and catch a train from there in direction of Sapporo.
Basics on how to travel from Tokyo to Sapporo:
http://www.japan-guide.com/e/e5306.html