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.
The word "plumber" dates from the Roman Empire. The Latin for lead is plumbum. Roman roofs used lead in conduits and drain pipes and some were also covered with lead, lead was also used for piping and for making baths. In medieval times anyone who worked with lead was referred to as a plumber as can be seen from an extract of workmen fixing a roof in Westminster Palace and were referred to as plumbers "To Gilbert de Westminster, plumber, working about the roof of the pantry of the little hall, covering it with lead, and about various defects in the roof of the little hall". Thus a person with expertise in working with lead was first known as a Plumbarius which was later shortened to plumber.