At the airport you follow the signs “railway station”. At the train station you can look at the departure board and see where and when trains leave. You buy a ticket from a ticket machine, or the ticket office, or using your app, or on the website. That is up to you. Then just get on the train, find a free seat, and travel. Tickets do not come with seat reservations, and those are not needed.
This is the procedure at pretty much every single airport in Europe that has a train station…