1. Travel by the airport metro. Airport metro ticket is not included in T-casual ticket (previously called T-10 tickets). The ticket price is €5,15. Operates from 5 AM to 12 PM daily, 2 AM on Friday and Saturday. The metro goes through both T1 and T2, with an advantage is that you don't have to change to buses, just change the Lines in the metro to get to where you want to go.
2. Travel by Aerobus, this bus only goes to the central square of Plaza Catalunya, then you will need to buy a T-casual ticket to continue on other vehicles. The advantage is that it takes only 35 minutes to go to the center, and it is very convenient if your hotel is in the center. Another point is the bus operates through the night. Price is €5,5/one way.
3. Terminal 2 has a train to Sants; if you have a T-casual ticket given by someone, you can take the train to Sants station and use that ticket to transfer to other metro/buses to get home. T-casual is €11,35.
4. Buy Hola Travel tickets; I don’t recommend this method because this ticket expires after 24h, 48h or 72h. For example, if you want to go to the suburbs for a day, the ticket will still be expired even if you didn’t use it. In my opinion, you can just buy a T-casual ticket and buy another one when you need to; it is available at any ticket machine.