Your question is so fun that I would like to give you some advice, but you have left out a lot of information in order for me to advise you fully. Most of the people who are designing shoes are established name designers or work for noted footwear companies. If all you want to design are shoes, you can learn how by taking an in person workshop or online course such as the ones offered at I Can Make Shoes. The link to their website is below.
If all you want to do are shoes, you would have time to also attend college and major in one of your other interests and get a degree for an additional career while you also make shoes. Designing them will not be enough, you will have to construct the shoe as well as design it and market it after manufacturing it, but you can learn how to do this. You may want to market your original styles to stores or manufacturers, so you will have to give or show them samples. But you have asked how to be only a designer which means creating/sketching the styles for shoes.
I would advise getting a job in a shoe store. You'll learn a lot about construction, brands, designers and the shoe industry. It may be a great place to make professional contacts who know the retail market and can advise you on your focused interest.
There is actually a website that tells you how to design your own shoes and possibly get them sold on line. This website is called Alive Shoes and I have left the link for it below. You can apply as a designer, design shoes and start selling the shoes. Speaking of on-line, there is a world of information and instructions on how to make shoes and slippers. Just do a search for this and you will be surprised at the many, many websites and videos there are that can help you with this. The sky's the limit !
It's not going to be enough to just draw different shoe styles, you will have to create the shoe. Become familiar with how shoes are made, the various materials and tools you will need to buy. It will be an investment. If you only want to design, you can sketch and send out your sketches to various companies to see whether or not they would like to buy your design for manufacturing and marketing.
I know this is limited advice but I do hope it is a little help and can get you started. I wish you all the best in your creative endeavors !
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