Buy second hand
Whenever you can, buy second hand! To save the planet from use of the energy, resources and ultimately carbon footprints that comes with producing a new item of clothing.
Buy natural fibres
If you need to buy something new, make sure you buy something made of natural fibres, such as cotton, wool, linen, bamboo... rather than synthetic fibres such as nylon, polyester, acrylic, spandex, lycra, viscose, elastin....etc. which are just different types of plastic. When you wash clothing made from synthetic fibres they will shed millions of tiny fibres, also known as micro plastics, that will go via the water ways into the ocean and eventually end up back in our food chain through the water we drink as well as the salt and sea food we eat.
Don't wash too often
The more you wash your clothes the more they will wear. So make them last longer by not over-washing. Wool for example contains lanolin, which have natural antibacterial and self cleansing components and shouldn't be washed very often.
Don't wash on high temperatures
Usually don't wash over 30 degrees, unless really necessary. For some items a cold wash is suitable as well.
If you have a number of items made from synthetic fibres there are wash bags that you can buy, that are designed to catch most of the plastic micro fibres before its being flushed out through the water ways. One of these are the Guppy Bag