Public transport in Sofia is cheap. There are three types of it - buses, trams and underground railway (Metro). We recommend using the Metro for your visit here, as it should be the easiest way for you to get to and from the venue.
The ticket machines at the Metro stations only sell single-ride tickets, at the price of 1.6 Leva, or 80 Euro cents. While most of them accept only coins, you can use paper money at some of them. Make sure you have banknotes of 2, 5 or 10 leva, as they do not accept banknotes of 20 and above.
Also, if you are traveling with bulkier luggage (if the dimensions exceed 60х40х40 cm), make sure to take another ticket for your luggage.
There are only two lines of underground trains in Sofia - Line 1 (red), and Line 2 (blue). Line 1 connects the districts of Lyulin and Mladost, while Line 2 connects the Obelya and Lozenets districts.
The conference venue, Generator Sofia is on the blue line. If you are coming directly from the airport, you should change at Serdika station (red) and take the train in the Lozenets(blue) direction. You will need to get off at station Vitosha, that's the closest one to Generator.
If your flight arrives late, your only option might be taking a taxi to the city center. You may want to avoid taking a taxi from the drivers right at the arrival section. On both terminals, there are regulated taxi stops. It might be better to use them. Take a look at the prices (usually located on back seat window), and make sure that the “TRANSPORT (leva per km)” price is not more than 1.00 lv.