The train connection between Helsinki city centre and Helsinki Airport takes about 30 minutes. There are two services: train I and train P. Schedules, ticket prices and routes are available at HSL website.
Single tickets are available from ticket machines, R-kiosks and other HSL sales points, or via the HSL app (recommended to download this, ticket prices are cheaper than from machines).
Bus line 615 and the Finnair City Bus operate between Helsinki Railway Station and the airport.
More information https://www.hsl.fi/en
If you want to get a private bus, you can ask for prices: