Trains are widely used in Poland. The network is very big and reliable, hence you are able to get by with this mean of transportation when traveling between cities. Bellow follows the link where you can purchase tickets.

If you prefer to travel by bus, there is one company both Kasper and I have used to travel inside of Poland. It is called Flexbus. The link to their right below, where you can see what routes they offer and where you purchase your tickets.

Ryanair is the biggest airline in Europe. And it happens to be a low cost airline with a significant network in Europe. With Ryanair you will be able to arrive to Poland from any major city in Europe, and also go visit many destinations in Europe leveraging Ryanair. Their ticket prices go as low as 20€ per ticket!