I was brought up to think that when exercising, it is best to wear something old and ratty that you wouldn’t mind ruining. At least that is what my grandmother taught me. It took me a while to learn how to dress properly for exercise, but once I did it became fun to dress for fitness. I used to think it is weird when a woman wears full makeup (I still think that one is weird) and matching outfit to the gym (I like to do this myself now). I don’t wear any makeup while working out but I do wear sunscreen and lip balm. When I am out under the sun I wear sunglasses and a cap to protect my face and eyes.
Looking nice in a flattering outfit has its psychologically benefits, can help motivate people to actually exercise in public, and will give you less excuse not to exercise. Dressing properly is also practical as it prevents injuries and discomfort. For example, it would not be smart to wear baggy sweatpants to working out because it might catch on exercise machines like stationary bikes. Wearing heavy materials will so prevent you from having a good workout because the fabric absorb your sweat will just make up feel more wet and heavy as you exercise.
Wearing clothes that fit (not too tight or too loose) will also be important because fitted fabrics that move with you and allow the full range of motion will help you focus on exercising better instead of thinking about how uncomfortable you are. Wearing fitted clothes are also helpful in yoga and Pilates where clothes might ride up during some of the moves. Clothes that are properly fitted will wick away sweat better, helping you to stay cool. Instead of wearing cotton, it is best to wear wicking fabrics to stay comfortable and help pull sweat away from the skin and evaporate quickly. If you are wet, you can get cold and uncomfortable fast when it is cold out, or overheated when it is hot out. Wearing layers are helpful as you can take off a piece of clothing when you are warmed up.
If you are running, a proper supportive bra is VERY important and will keep you from feeling breast pain (especially if you have large breasts or are breastfeeding!). Proper socks will keep you from getting blisters, and the right shoes will support your feet and prevent injury as well.
My fitness journey started with t-shirt, a tank with shelf bra, capris, and running socks. Once you start doing more, then you can build on what you have. I rewarded myself with new fitness apparel shopping about every 5 pounds that I lost or a new goal achieved, so now I pretty much have what I need.