If you really want to work out the fare system, what you need to do is pick up a schedule for the line you want to ride, which are available at major interchanges and usually on the bus itself. This will list out the bus route and the zones they are divided into. Count the number of zones you're passing through and match it to the fare table, some lines even have handy-dandy charts where you can just connect X to Y and find your fare Z at the intersection. And one more bit of complexity: air-con buses are a bit more expensive than (rare) non-air-con buses.

But trust me, you don't want to: just buy an ez-link card instead. If you're staying in Sing long enough to learn to use buses, it will probably pay for itself pretty fast (15% discount for all travel), and if you aren't staying... take taxis.