With no public transport to and from the city, your only choice is to use the local taxi companies - there are 4, approved by the airport authorities as reputable to wait at the taxi ranks. The fare into town should be around 10 to 15 leva. If you are driving, the car parking fee is apply 2 leva per hour with only the first 15 mins free of charge. Reasonable long term rates - details here.
A tourist info centre inside the terminal provides free maps, leaflets and tourist guides