Just a quick note on the Relay I used for my project.
Initially I thought I'd be clever and buy components to build my own relay - big mistake as its so much cheaper to buy ready made unless you intend setting up a factory! I bought a pack of 3 relays for under £7 from Amazon, so you're looking at just over £2 a relay that will switch up to 2A and has power and relay state LEDs built in.
There are 3 terminals to connect so you will need jumper wires (female to male, or just female as I stripped the ends that screw onto the relay).
Positive - connect to a 5v pin on the Pi
Negative - connect to a GND pin on the Pi
Trigger - connect to GPIO18 or the GPIO of your choice
See my post on setting up a python script to activate the relay.