With the Swiss Travel Pass you still need a ticket to go to the Jungfraujoch, as the STP only covers public transport, not mountain railways, With the STP you get 25% off on the Jungfraujoch ticket. You can buy that ticket on the jungfrau.ch website, or at the station in Interlaken.
Best is to just buy it at the station in Interlaken once you know the weather forecast. You can dan also reserve your slot. There are no seat reservations on that route, but you can buy a priority boarding slot that permits you to skip the queue for the last segment.
