My DD and her teammates wear sport bras and no underware but they all do have the underwear that GK sells but only 1 girl wears them at her moms instance. The others go like you would for a bathingsuite. Im sure that will change when they get their periods.
The little ones generally go without underwear. A girl your age should wear underwear. You can wear any underwear that will not show. For example a string bikini that is high cut. You could by the GK briefs but they're expensive for everyday use.
Do you have your leos yet? You could try a few different pairs on under your leo until you find what works. But definitely, at your age you want to wear something under your leo.
I wear a bra from GK and underwear from GK. For meets, the same thing. But this year my leo doesn't fit right, so i don't know what i will do, haha. I will just have to pull them up REALLY high, because our leo is high cut and rides up really bad, lol.