Hi Shubham, You can fly to Jamnagar Airport or Kandla Airport and then take a taxi to Dwarka and Somnath.
There are trains available too, but given your distance I recommend flights, although if you have that sort of time you can even take a road trip from Kolkata, if feeling adventurous! Have a fun trip!
Here's an article that you can go through for more details:
First Try to reach Jamnagar or rajkot by train Or flight. now you can take bus, train Or cab from rajkot or Jamnagar to visit Somnath or Dwarka. Dwarka to Somnath you can take train, bus or car there is two way available one is by porbandar and second one is by rajkot, Jamnagar. you can check on map and choose your best.