Pack some nice homemade food, cause this isn't a short trip !
There are direct flights but no direct trains for this journey. Mumbai (Maharashtra) to Port Blair (Andaman And Nicobar Islands) distance is 2282 km and you can get there by flight in just over 4 hours and 40 minutes or by train + flight fares starting from ₹ 4101.
The weather is amazing right now. You'll have a great time.