Hey Saumiabee, If you are about to start the biking in the mountains, I will recommend to start with Kumaon region. Depending on the number of days in hand you can pick places from Chaukori, Binsar, Kuasani region. Then go for a ride in around Manali region in Himachal, ride towards Rohtang and be back. If you are really comfortable with these then next one you can plan for Himachal up to Kinnaur region and test. Finally, go for Spiti after couple of more rides :)