Nothing is required to travel by bus, however buses have very limited routes.

To travel by a bus, approach a bus stop (the green bus symbol on the mini-map) and click on it.
A list of destinations will be displayed on the right.
Click any of these destinations and a picture of the bus stop at that destination will be displayed. Click this to travel to the location.

Bus stops are always in pairs on opposite sides of the road. The destination you arrive at will always lead back to your point of departure, but can also have extra stations to travel to, so it's worth checking the bus stop when you arrive.
If more than 2 bus stops exist, then the other pair(s) are for different bus routes.








Example of screen after clicking on a bus stop