You can travel from India through any of these airlines: Oman Air, Jet Airways, IndiGo, Air India, SpiceJet and Air India Express. There are flights from Kerala (Kochi, Kozhikode and Thiruvanthapuram) that fly directly to Muscat International Airport.
The easiest way to reach Nizwa is to fly to Muscat International Airport.
After arriving in Muscat International Airport, you can either rent a car and drive to Nizwa or hire a cab. In order to rent a car and drive in Oman, you will need an International Driving Permit. You can rent cars from Speedy Oman, Sixt Rent a Car and Arabia Cars. Alternatively, if you want to hire a cab, there are plenty of taxis at the airport.
The Oman National Transport Company (ONTC) operates buses to and from Muscat twice a day. The journey takes around 2.5 hours.