Let's start with the easiest possible project: Installing a new shower head is as simple as it gets. Typically, all you have to do is unscrew the old shower head from the shower arm (the pipe coming out of the wall), clean off the threads on the shower arm and wrap a bit of Teflon tape around it, then screw on the new shower head. It's really that simple (and if you don't believe me, check out the video above from Dummies.com). Do read the installation instructions on the shower head you buy, though. Some are designed not to require Teflon tape and can actually leak if you use it.
Plumbers in Australia have licensing requirements that differ from state to state but it is generally accepted a 4-year apprenticeship with a further minimum experience of 2 years (6 years total) and a further curricular requirement as a benchmark for licensing. Licensed plumbers are also expected to maintain minimum relevant training requirements to maintain their plumbing license