Travel / Buying train tickets - easiest / cheapest way to buy them in Poland? [4]

Hello all
I am coming to Poland in October and will be travelling from Gdansk - Torun - Warsaw - Zakopane - Krakow

Can I buy tickets at the station on the day of travel?
Or should I buy them online? If I buy them online is the ticket then on my phone to show to staff on the train?

Just want to know the easiest / cheapest way to buy them

Thankyou all so much