White necklaces used to mean one thing, a single strand of something trying to pass as pearl, usually with a clasp that gave up within a month. That's shifted a fair bit. There's much more range now, enamel pieces, bone and shell effects, white stone beads mixed with gold, and the quality at the ordinary end has genuinely improved rather than just the prices going up.
If you own none of these, start with a simple graduated bead or a single row of faux pearl in a medium size, something that sits just below the collarbone rather than choker height or down at the chest. That length works with crew necks, shirts open a couple of buttons, and most dresses, so you're not starting with something that only does one job. Once you've got that, a pearl or shell choker is a nice second piece because it changes character completely depending on whether your hair's up or down. Worn and looked after properly, a decent white necklace should hold its colour for years rather than months. The thing that kills them is moisture and friction more than anything else, so it goes on after perfume and hairspray have dried, not before, and comes off before you shower or sleep in it. Wipe it with a soft cloth after wearing if it's been against skin for a few hours, especially in summer.
The difference between a necklace that lasts and one that turns yellow or chips within a season usually comes down to what's under the white. Pick it up and look at the drill holes or any chipped edge, if you can see one, cheap resin or plastic often shows a dull grey core rather than white all the way through, which tells you it'll wear patchy. Good faux pearl has a slight weight to it and a cool feel for a second before it warms in your hand, proper glass does this more than resin does. Check the clasp with your fingers, it should click shut firmly and not spring open if you tug gently, and the metal shouldn't feel light or hollow. Look at where beads meet findings too, any visible glue or a seam that catches your nail is a sign the finish was rushed.
Length is really the only fit decision here, but it's worth getting right because necklaces drag outfits the wrong way more than people expect. Hold the piece up against your own neckline before buying, not just on the packaging image, since the same length sits completely differently on a high neckline than on something with a low scoop. If you've got a fuller bust or a shorter neck, go slightly longer than you think so it doesn't disappear into the chest or sit awkwardly tight. Keep white necklaces stored flat or hanging separately so they don't tangle and scratch against darker metal pieces in a jewellery box.