If you wear shorts, I would imagine regular underwear would be fine, but if you go without, you may want to invest into a pair of the underleo underroos, just to see how they work for you
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.