With all the talk about the environment, climate change and sustainability, it's natural that environmentalists and fashion influencers alike are talking about how to reduce the environmental impact of a commodity that uses vast amounts of resources and produces an undue amount of waste: clothing. While "reduce, reuse, recycle" is an oversimplified way of looking at one's environmental impact that focuses on individual responsibility and ignores larger factors at play, a sustainable society relies on reduced individual demand for clothing to some extent, so it's not a terrible starting point.
Reducing your clothing purchases is a good way to lessen environmental impact--while some recommend buying used or from more ethical "slow fashion" brands, many have argued that this practice has gentrified thrifting and pushed out people who buy used out of necessity, and even a more sustainable product has an environmental impact when it is discarded. It's better to reduce one's clothing consumption, such as by buying well-made pieces that are unlikely to fall apart easily and will last a long time, rather than cheaply-made pieces that are likely to be worn down quickly. It's also helpful to purchase versatile pieces that go with other things in your wardrobe, so that you will get a lot of wear out of them over the years and will be unlikely to throw them away once you get tired of them after a few months, as you might with something that is in style but doesn't go with your other pieces.
Personally, I have applied this logic to my university "capsule wardrobe" --most of my clothes are either solid colors, or classic patterns like stripes that don't get old. It helps to choose a few favorite colors (in my case forest green, burnt orange and red), then mix them with neutrals (black, white, gray, tan) and a few classics like blue jeans and a plain black dress. I also have a few less-versatile personal favorites (the penguin jumper, the periodic table Christmas jumper, and the punny hedgehog shirt), but the vast majority of the pieces suit multiple seasons and moods.
As for reuse, there are plenty of shops and websites where you can resell old clothes, although these have been criticized due to sellers reselling used clothes at a marked-up price. However, selling used clothes honestly and at a reasonable price is legitimate. It's also fun to trade clothes with friends and family who wear a similar size--a lot of my favorite outfits used to belong to my mom, who's given me a lot of her old shirts.
As for recycling clothes, there are a number of ways to incorporate old clothes into new projects. It's fun to make t-shirt tote bags and there are even tutorials on how to make quilts and blankets out of old shirts.
Although it's near-impossible to have a completely sustainable wardrobe in our current society, finding creative uses for old clothes (and buying fewer new pieces) is certainly a step in a more sustainable direction.