Updated: Jul 2, 2019
By far the best way to stay green while shopping for clothing, or anything else, is to buy second hand. Back when I was growing up, I know buying from thrift stores was not something to brag about, but friend, times have changed. Instead of going shopping for fun and buying all the things you like, try to find what clothes you are in need of (white sweater, black cami, light jeans) and before you buy them check Good Will or any other local thrift store. If they do not have what you are looking for, by all means get what you need at your favorite retail store. Clearly undergarments always need to be bought new. You can find some stores and undergarments that are made in environmentally responsible ways.
Brands that focus on sustainable clothing are growing in size and popularity. Companies are beginning to focus on products that are earth and worker friendly. Some examples of "green" clothing companies are Alternative Apparel, PACT, People Tree, Fair Indigo, Maggie's Organic, Patagonia and Urban Renewal. These companies focus on recycled materials as well as sustainable materials that limited the use of water and energy to produce. These companies typically pay their workers a livable wage as well as providing good working conditions. Some of them are even made in the US.
There is also a list on ThredUp: https://www.thredup.com/bg/p/top-sustainable-clothing-brand
If you are not looking to buy online or switch up your shopping habits significantly, then keep some things in mind. While shopping you can check the tags for sustainable fabrics such as linen, hemp, bamboo, alpaca, organic wool or silk. Although organic cotton is better for the environment than conventional, it still can use a significant amount of water to produce. So going 100% organic cotton may not be the most Eco-friendly way to buy clothes. You can also look to see if items have been "reworked" meaning that they are made from recycled clothing. Happy shopping!