The thing nobody tells you about boots this time of year is that they'll look brilliant in the shop and then let in water the first proper rain, so test with your hands before anything else, press the sole where it meets the upper and check for any gap, flex the toe to see if it creases naturally or just cracks. Once you've got a pair that passes that, autumn boots genuinely change how much use you get out of the rest of your clothes, because they cover you for months where trainers get soggy and sandals are plainly wrong.
If you own none yet, start with an ankle boot in a flat or low block heel, in brown or black leather or a good leather look, nothing too pointed and nothing too chunky at the sole. That one pair will do the most jobs, which matters more than finding something exciting straight away. From there you can think about a knee high for the colder, wetter stretch and something with more grip if you're walking a lot rather than driving everywhere. Early autumn, when it's still mild but the mornings have turned, is when ankle boots come into their own, worn under jeans or with bare ankles and a skirt while it's still warm enough to risk that. Once the weather properly breaks, into November and the darker evenings, the knee high or a chunkier boot takes over, especially for anything outdoors or anywhere you'll be standing about. Think school runs, weekend markets, a walk that involves actual mud, versus an evening where you want something smarter and don't want wet feet ruining it.
On the pairing side, these work hardest with the clothes you already have rather than anything new. A straight or slightly flared jean sitting just at the top of the boot looks better than one bunched up inside it, so try that on in the shop with the actual boots if you can. Midi skirts and dresses want a boot that doesn't disappear under the hem completely, so a bit of heel or a visible shaft helps the proportions. A jumper that comes to mid hip balances a knee high boot nicely, while a longer coat needs a boot with enough leg showing that you don't look like one solid block. Check the zip while you're trying them on, pull it right up and down twice, and check the inside lining with your fingers for any rough seam, because that's the bit that rubs after an hour, not the bit you notice standing still in a shop mirror.
These suit anyone who's on their feet more than they'd like between September and the first proper frost, and wants one decision sorted before the rest of the outfit even gets a thought.