We have two routes from Delhi to Shimla. The best route is via national highway via NH 44 and NH5, which is the fastest. The distance between the places by this route is 300 kms. You should reach your destination in 9 hours. You can make a stop or overnight stay at Kullu.
September to November: September is the end of the monsoon season and the winter just sets in. Winters are cold in Shimla. The town receives its first snowfall around December last. Do carry some heavy woolen clothes as the temperatures can drop down below zero. December to January is the time when you can enjoy snowfall. This is the ideal time for adventure enthusiasts who want to go skiing in Kufri. March to June: The summers in Shimla are pleasant. The temperatures during these summer months usually vary between 10°C to 25°C. The valley is covered with flowers and is a sight to behold. July-August: Monsoons in Shimla are not an ideal time as there is heavy rainfall accompanied by landslides and traffic. However, this is also the time when you can get the best bargains on hotels.
There is another alternative route vis NH9 which passes through Patiyala. This is the longest route of the three. It is about 481 km and takes 10 hours to cover the distance.
The roads like any other mountain terrain are steep and full of curves. However, with professional cab services, there is nothing to worry about. Book a cab and enjoy your road trip.