Tainan is not a very big city and it has no metro. A good option to move around is the bus (some of them are free), but sometimes it's really challenging if you cannot read Chinese. The taxi is an expensive option if you plan to move a lot. So the best option, if you got a car license, is to rent a bike for about 400NTD (9€) a day. The city is not very complicated, so you'll be able to move easily reading a map.
Take care, the traffic is crazy! Sometimes looks like there's no rules!
The first thing you should do if you are planning to go to Tainan is to get one of the passports that the local Government offers to travelers. For 999 NTD (22€) you'll get a two person ticket for a night in a hotel (or great discount if it's a expensive one), free tickets for historic sites, two free meals in two great restaurants (really!) and many other gifts and discounts for several shops along the city as bike rentals, bus, snacks, tea shops...
The definition of pub in Tainan seems to differ a little bit from the regular one. Most of the bars using this word within the door sign are actually red-light bars. So, if you are just looking for a cool place where have a beer, try to avoid the "pubs".
