Also, you need to know everything which will directly feed through the switch. It might be just the starter solenoid and the ignition, but it could be a whole lot more.

As for where, that has two answers: where the breaker or fuse is physically positioned (your preference) and where it is located in circuit.

Your electrical system will be most efficient if you have a distribution block, which feeds all. The alternator should reference system voltage from this junction also. Typically, the various circuit feeds go from the junction to a fuse block, then to the circuits. Some like a master breaker in front of the junction (distribution block).