Here's a quick primer. Depending on the type of bus, you can have either 8 or 16 devices on the bus. The controller itself is a device, so you're left with another 7 or 15 available. Each device has a SCSI ID, usually set by jumpers (the controller's ID can often be set in its own BIOS). There musn't be 2 devices with the same ID. The controller often takes the highest ID (either 7 or 15 depending on the bus). Boot disks will often have ID 0; the CDROM is often ID 6. IDs go from 0 to 7, or from 0 to 15.
Each bus must be terminated at both ends. The terminators are essentially dampeners of signals that get past all devices, and absorb them so they don't get reflected back on the bus. The simplest are passive terminators which are resistor packs that shunt each signal line to the ground. They aren't suitable for high speed buses - practically over 5MB/s you will need to use active terminators instead. Many devices, including controllers, have terminators built-in which can be switched on or off. Usually the controller is at the end of the bus so it will need to have termination turned on in that case (there are configurations where the controller sits in the middle of the bus, and then it must not be terminated). And the device at the other end of the bus must either have its terminator on as well, or else you have to install a dedicated terminator after it on the bus.
That's it in a nutshell.
Some crude diagrams:
Controller (terminated) -----(bus)-----Device1-------Device2-------Device3---Terminator
or
Terminator---Device1-----Device2----Controller (not terminated)-----Device3-----Device4--Terminator
Each device including the controller must have a separate ID, but they don't need to be placed on the bus in any particular order of IDs. There is no equivalent of the "cable select" as for IDE.
I/O, I/O,
It's off to disk I go,
With a bit and a byte
And a read and a write,
I/O, I/O