To reach the Estrel using Berlin's public transport facilities, you will require a single journey ticket for the AB zone costing 2,80€ (Exception: Please note that when travelling from the Berlin-Schönefeld Airport SXF, you will require an ABC zone ticket costing 3,40€).
You can purchase tickets at the BVG service counters, the BVG ticket vending machines located in the S-Bahn and U-Bahn stations or on busses directly from the driver (please have change ready for bus drivers), as well as in the BVG app available for Android and iOS called "FahrInfo Plus".
Website of the BVG (Berlin's public transport company) BVG App for Mobile Tickets (Android & iOS)