Under public transit is where the tours tab is.
You need to build a sightseeing bus depot before you can build lines.
Make sure you have lots of parks and your city is a decent size before creating
the lines or they will overlap or be very short. This is not good if you want to keep
the lines after getting the achievement.
Simply create 15 lines and the achievement is yours. Start and end your lines at major transit points like ferries or airports.