Getting from Helsinki airport to Tampere is easy, but it takes some time. Because the price vary a lot (between 25-50 EUR), depending on the time you are traveling, I would suggest to book the train in advance.
The easiest way to get from Helsinki airport to the main railway-station is to take the airport-bus. It goes every 20 minutes, cost 6,60 EUR and takes 30 min. There is free wifi on board. For those of you who dislike to take a bus because there’s no toilet there, there’s a toilet at this bus! However I didn’t try or see it, but at least there is one.
We had booked a comuter-train and didn’t know we had to sit in the front, but luckily we paied attention to the speaker and changed train at the next station. Be aware of this!
The train has a toilet in the back, but I am not sure if it worked or not.