Oh I love Finland! It is a wonderful country, where people value silence and personal space. Don’t be fooled by the Finnish cold exterior, they are amazingly kind people. I highly recommend visiting if you get a chance!
Arrival in Vantaa Airport: There is a train from the airport to the center of Helsinki. If you download the HSL app you can purchase the ticket and any public transit ticket (for Helsinki). You can use your foreign card on it and it is in english as well. It is quick and easy to get to and having the ticket on your phone means no actual paper ticket that could be lost.
In Central, you have their trams, buses, and metro. If you buy the AB pass you can go pretty much anywhere in the main part of Central. The trams all go past the Central Railway station so if you get lost it is easy to find the way back to the main part of the city.
SIM Cards, you can get a rechargeable SIM card in Kioski. I have one that I keep year round as I am there visiting my boyfriend often. I have found that to recharge it prefers my Visa credit card over my debit card, but you can just go into Kioski to have it topped up too.
VR is the app you use to buy train tickets outside of Helsinki. It also has a QR code reader, and it gives you pretty good information on how long it will take to get there and all the stops the train will take.