The straight sections of plumbing systems are called "pipes" or "tubes". A pipe is typically formed via casting or welding, whereas a tube is made through extrusion. Pipe normally has thicker walls and may be threaded or welded, while tubing is thinner-walled and requires special joining techniques such as brazing, compression fitting, crimping, or for plastics, solvent welding. These joining techniques are discussed in more detail in the piping and plumbing fittings article.
If you have questions about what parts to buy for your fixtures, the folks at the hardware store will most likely have an answer for you. Come equipped with the brand and model of your fixture and, even better, some pictures. They'll point you in the right direction. And if at any point you feel like you're in over your head, call a plumber. Even if you think you have the skills to do the job, there might be codes involved and you often need a permit.
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