My first pair of slippers came free with a magazine, basically a bit of felt folded in half, and within a week the sole had gone through on the kitchen tiles where I stand doing the washing up. What they taught me is that a slipper needs an actual sole, not just a lining, or it's not really a slipper, it's a sock with ideas above its station.
If you've got none in the house right now, start with a simple mule style slipper with a firm insole and a rubber sole that goes properly round the edge, not just a thin strip down the middle. Pick them up in the shop and press your thumb into the footbed. If it springs back straight away with no give at all, that's foam that'll go flat in a month. You want something with a bit of resistance, a bit of bounce, that tells you there's real cushioning in there rather than a layer of fabric pretending to be cushioning. Check the sole bends where your foot bends too, not halfway along like it's been designed by someone who's never actually worn a shoe.
Colour wise, go duller than feels exciting. A bright white or pale grey looks lovely on the shelf and then you wear them twice across your own hallway and they look like they've been dragged behind a car. Charcoal, a deep berry, a proper chocolate brown, those hide the scuffs and the general grime of a slipper's actual job, which is getting walked on by someone who's just finished doing the bins. Anything with a slight texture, a ribbed knit or a brushed finish, hides wear better than something flat and smooth, which shows every mark like it's been personally offended by your floor.
As for what they go with, slippers are doing their best work with the rest of your loungewear rather than against it, so a longer robe or a baggy knit jumper worn as a dressing gown substitute looks properly put together over a simple slipper, where a dainty little ballet style one can look a bit lost under anything heavier than pyjamas. I wear mine with thick joggers most mornings and they disappear completely into the outfit, which is exactly what you want from a slipper, not a statement, just something warm that isn't falling apart by February. The one thing I'd avoid is matching the colour too closely to your dressing gown, it starts looking like a matching set you were given rather than clothes you chose.
Try them on wearing the socks you'd actually wear at home, not bare feet, because a slipper that's snug barefoot in a shop will pinch the second you add a sock on the sofa at nine at night.