If you want to use public transport from the airport to central rail station/bus terminal "Serdica" you have to take bus No 84 and change it at hotel Pliska bus stop to buses No 213, 214, 305 or 313. The ticket (must be purchased in advance at bus stops) costs 50 st. (25 euro cents); each large luggage must be taxed extra.
If you decide to take a taxi, please, use the desk in the arrival lobby to book a cab.