I bought our return tickets online from the Aerobus site (www.aerobusbcn.com) and printed the confirmation to exchange at the Aerobus ticket booth in the arrival area or to the driver of the bus as indicated in the site. I have paid €10.20 for return for myself and daughter. A small receipt was printed for our return journey and I just had to keep them to use again on the return leg.
The bus is very easy to find. There is a big sign outside the arrival area in Terminal 2 where you can board the Aerobus. Waiting time is not much. There are buses leaving every 5 minutes. You can check the timetable from their site.
It stops in major areas in the city like Plaza Espana, Gran Via-Urgell, Plaza Universidad and lastly, Plaça Catalunya. And if you're planning a trip to Barcelona anytime soon, consider getting hotels around those areas so you can just alight on any of the major stops and walk towards your hotel which we did in Plaça Catalunya.
You can take other modes of transportation like taxi and public bus from the airport to the city but we preferred to use the Aerobus service because it has wifi, purchasing was like a breeze (and you can purchase directly from the driver as well), there is a wide luggage compartment inside the bus itself so you can keep an eye on your belongings as compared to other airport bus where you stash your luggages in a special compartment outside.
Using the Aerobus service is one of the cheapest, most comfortable way of getting to your city centre destination in Barcelona.