The single most useful thing I can tell you is that black trainers are never really just black. Hold two pairs next to each other and one has a warm, slightly brown cast while the other is flat and cold, almost blue in certain light. It matters more than you'd expect once you're trying to match them against black trousers or a black coat, because a mismatched black is somehow more obvious than two completely different colours sitting next to each other.
If you've got none, start with a plain low top in a smooth leather or leather look, white sole kept slim rather than chunky, no mesh panels and no metallic bits. That version goes under a dress as easily as under jeans, and it won't date the way a trend shape does in about eighteen months. Save the chunkier, more directional styles for a second or third pair, once you already know a plain black trainer earns a daily spot and you fancy something with a bit more opinion.
On fit, go half a size up from your usual high street size if the upper is leather or a stiff synthetic, because those materials don't give the way canvas or knit does, and a snug leather trainer stays snug forever. Canvas and knit uppers soften and stretch with wear, so true to size is usually right there. Check the heel first, not the toe, slip them on and walk a few steps without socks if you can, because a heel that slides even slightly will rub by the end of a day and no amount of breaking in really fixes that. Width matters more than people admit, if your foot is wide, a narrow pointed trainer shape will pinch across the little toe no matter how good the length feels standing still.
What they go with is almost everything, which sounds like the obvious thing to say but is genuinely the point of owning them. A soft, slightly drapey black trainer with a straight leg jean looks deliberate rather than lazy. Under a midi skirt they read as a choice, not a compromise, especially if the skirt has some movement to it. With tailored trousers they take the formality down a notch without looking like you gave up halfway through getting dressed, and against a plain t-shirt and blazer they do most of the quiet work of making the outfit look considered rather than assembled in the dark, which, some mornings, it has been.
Best for anyone who wants one pair that works from a school run to a casual dinner without a second thought about whether it matches.