there will be no 3G on my mobile
You can buy a prepaid SIM card in the airport (in the arrivals section, after the customs check, there's a shop which sells newspapers, magazines, etc, I think it's called Relay).
when i arrive to Kaliska bus station how can i get a Taxi for my final destination ?
There's a taxi rank outside.
is it ok to use dollar for both Modlin busses and taxis ??
You can't use dollars in taxis. And are you arriving at Modlin or Chopin?