We highly recommend you Ljubljana Pub Crawl for your first night. A lovely English-speaking guide will lead you through the night streets.
She will take a good care of the whole group introducing you to the drinking culture and customs plus she may teach you some pick-up lines that you can use later on.
Throughout the night you will visit several bars and pubs and the adventure will end at the liveliest nightclub in the city.
Keep in mind that Slovenians usually go out to drink around eight in the evening and go to clubs after midnight. The clubs are open until 6 in the morning.