For this problem, you can listen to the advice of Hucai, an experienced sportswear manufacturer.
*Decide what kind of sportswear you want? Why is it different? What work do you want to make? What is your brand? (Cost: your time)
*Able to describe the clothes you imagine (use drawings or refer to existing clothes) (cost: your time)
* (Optional) Test whether this is what people want and complete the concept. Share them with friends, professionals and strangers and get feedback. (Cost: your time)
*Determine the manufacturer, price, whether it can provide you with customized solutions and transportation methods?
*Determine the sales method you want, you may also need the design of the website, product photography, logo and/or label, shipping package, email, brand manual, etc.... If you can, please do it yourself or a paid professional. . (Cost: Hundreds/thousands of dollars in your time and setup cost or hired work).
*After the product and everything are in place, you can start selling.
This is an easy way to get you started. Then you need to start marketing and get people to notice and buy your stuff.
Business owners, if you have wholesale or customized fitness clothing needs, you can ask hcactivewear for help. They provide private custom solutions for brand owners, Internet celebrities, and wholesalers.