lol, there isn't some arbitrary thing you can say that will get companies to sponsor you

but using your social media (if you have a lot of tennis players that follow you), playing in tournaments/leagues/schools/ or anywhere that gets a lot of exposure. Being a member of a large tennis center, pro, coach, and any other position that gets you exposure to players. I don't know your life, but a quick example. I love Prince. I have supported their brand through purchasing quite a few frames/strings/clothes/grips/bags. I post High definition pics and write reviews on their products that I have spent my hard earned money on. I use my social media to post pics of racquets and spread the word about upcoming products in my leagues/tennis centers. I have done something for their company, and they can actually look at my social media, and see results (that people are seeing their brand). If you were Prince, and two players sent you a letter asking for a sponsorship. One is me, and the other is someone who just sent an email asking for free stuff. Who stands to get a sponsorship ?
Good luck with the sponsorship, and spend some time thinking of what you can do for the company that chooses to sponsor you. (Sponsorship is a business relationship, tennis racquet companies are in business to sell racquets) One last example, instead of saying, I did a sponsorship for this company, you could have said, I love these shirts I get from Bidubadu, they allow me to play tennis and not be soaked in sweat at the end of a match. They are high quality and look awesome!!!

check em out! Here's the link!! (I realize that is a very cheesy ad sounding thing, but it's 4:45 am here and I just woke up. But you get the point. Whatever company sponsors you, do your best to help them build their brand!!! Who knows, one day, you could start a business with your fellow teammates selling strings, and if you sponsored juniors, wouldn't you want them to tell people how great your product is?) Good luck!!