I use Cage Free String, Kitty String and Project 44 String. So far the Project 44 string has been the best for slack tricks and it is really good value.
Here are good strings you can get on yoyoexpert, C3Yoyodesign string is really good and is probably what I would recommend for beginners.
don’t spend more than like 50 bucks on your first yoyo. It’s going to get dinged up, and a 50 dollar yoyo plays just as good as a 100 dollar one. If you stick with this hobby for any amount of time you’ll have plenty more yoyos later.