Wanna look sexy? Help?

I'm 15, and I cannot yet drive myself around anywhere. But I absolutely wanna look good for my girlfriend. My mom loves the cheap Walmart bra's and underwear. And she's against any high brand things. Help me!

Edit: wow, rude much lol. Like everyone on here doesn't want to feel good in what they are wearing. I just want to impress my girlfriend is that such a bad thing?