It depends on a lot of things. Which part of the world? In Europe big cities all have the tourist part and the “old town” part. If you are there sightseeing just get a tourist map and start exploring.
You can usually get an equivalent of a tourist day tickets, which will give you unlimited access to public transport, sightseeing buses and some attractions/museums. Just pick up the one which tickles your fancy and usually you can get it from any airport, big train/bus station when you arrive.
If you are there to party and check out the night life I would strictly stick to popular spots.
For any other interests (hiking, historical spots, etc) you can find a city specific list of recommendations online.
Also it usually helps to google which tourist scams are popular in the region to avoid them.