Hi Meetu,
Assuming that you wish to cover entire Spti Valley on your trip, the best route to take is from Delhi to Manali then from Manali to Lahaul - covering Chandrataal, Kaza, KIbber, Tabo, and Nako. And then from Nako to Reckong Peo to Rampur and then to New Delhi.
This entire trip should take at least 8-10 days, if you want a relaxed stay.
You might wanna read the following article for a better insight on your itinerary and the best places to visit.