The price of a single ticket is 1.00 lev (~ 0.50 €) only paid in the local currency. There are also 10 (8.00 lv) tickets for one passenger, combined ticket for subway and ground transport (1.40 lv), daily cards (4.00 lv), valid for all lines, and 5-day cards (15.00 lv). Tickets, daily and 5-day cards can be bought at most newspaper stands especially ones adjacent to public transport stops. If you can't find any, you can also buy tickets from the driver if he has any available, though this is not guaranteed. Punch a ticket immediately after you enter the vehicle. 

If you are traveling by tram with a large suitcase or backpack, be sure to buy a separate ticket for your bag/case or you might be fined by authorized inspectors! The fine in case you travel without ticket is 20.00 lv (~10.00 €).