The MTA currently runs 3 Select Bus Service routes in New York City: M34/M34A SBS, Bx12 SBS and M15 SBS.
They also have non-SBS versions of these routes, so make sure you take the correct bus. Select Bus Service buses light up in blue.
The SBS and non-SBS buses stop at different stops. Sometimes they are right next to each other, sometimes you have to walk a little.
SBS operates on a proof of payment system. You must pay your fare before your trip at the fare collector machines at bus stops, either with a MetroCard or with coins. You can board the bus through any door. MTA fare inspectors may ask to see your receipt, but your ticket is not checked when you board the bus.
Each receipt is only valid for one trip in the direction of the machine that issued it to you.
If you change your mind and decide to take a non-SBS bus instead, you can board through the front door and hand your ticket to the driver.
You still get a transfer to the subway or a local bus. If you paid your fare with a MetroCard, your transfer is on the MetroCard. If you paid with coins, board through the front door and ask for a transfer from the driver.