Eyepieces accumulate faster than they are used, and most owners end up with two they reach for and four they do not. This page is about arriving at the useful set deliberately rather than by accumulation, which is cheaper and works better.
Start with two, not five
A low power eyepiece giving a wide field for finding things, and a medium power one for general viewing. Those two cover most of an observing session, and adding a barlow doubles them into four magnifications for the price of one more piece of glass. Buy the third eyepiece after a season, when you know which gap you keep hitting.
Choosing the focal lengths for your instrument
Work out the magnification each gives in your telescope rather than shopping by focal length. Aim for a low power near your maximum useful exit pupil, around five to seven millimetres, and a high power near two times the aperture in millimetres. In a 300mm f/4 instrument that means roughly 25mm and 4mm.
Storage, which is not a luxury
Eyepieces are precision optics used in the dark on damp grass. A small case with foam cut-outs keeps them clean, keeps the caps with them, and stops the routine disaster of an eyepiece rolling off a table onto gravel. Any lined camera case does the job perfectly well.
Questions people ask about telescope
How many eyepieces do I need?
Two to start and three eventually. A barlow doubles the set cheaply, which is why two eyepieces plus a barlow is such a common starting point.
What eyepiece sizes should I buy?
Choose by the magnification they give in your telescope, not by focal length alone. The same eyepiece behaves very differently in a short and a long instrument.
Do I need an eyepiece case?
Not strictly, and it is cheap insurance for optics used outdoors in the dark. Any foam-lined case does the job.