Language
The main language in Finland is suomi (Finnish). Other official langugages are ruotsi (Swedish), mainly spoken in coastal areas and saame (Sami), spoken in Northern Lapland.
Finnish people learn foreign languages at school, so expect most people to speak English, either fluently or pretty well.
Commonly used Finnish words and phrases
With these words and phrases you will get pretty far. Finns appreciate any attempt of using the language, so don't worry about pronunciation.
Hei = Hello/Hi
Hei hei = Bye
Moi = Hi/Hello/Bye
Huomenta = Good morning
Hyvää päivää = Good afternoon
Hyvää yötä = Good night
Hyvää iltaa = Good evening
Kiitos = Thank you
Ole hyvä = You're welcome / Please
Anteeksi = Excuse me / Sorry
Kyllä = Yes
Ei = No
Yksi, kaksi, kolme = One, two, three
Sauna = Sauna
Vesi = Water
Kahvi = Coffee
Olut = Beer
Kalja = Beer
Ruoka = Food
Mitä tämä on? = What is this?
Missä on...? = Where is...?
Paljonko maksaa..? = How much does it cost?
Terveisiä! = Greetings! / Best regards!
Small talk
Finns are generally not proficient with small talk. If you ask them "how are you", they might take it literally and tell their whole life story :D