If you've lived in Finland less than ~2 years, you'll likely need a prepaid SIM instead of a monthly plan.
Visit any R-kioski, K-market, or airport kiosk and ask for a prepaid SIM. Options include Telia, Elisa, DNA, and the budget DNA-owned Moi.
Choose a package. Telia offers ~€0.99/day usage-based or 31-day unlimited. DNA Super Prepaid includes a cheap €19.90/month internet option.
Activate the SIM per the operator's instructions. Top up at the same kiosk or online — note that even 'all-inclusive' plans may charge extra for landline or international calls.
Source