On that note, lighting matters. iPhones are awesome and powerful, but they don't handle bad lighting very well AT ALL. So unless your dark photo is, like, a selfie with Justin Timberlake, I suggest starting over and finding better lighting. Windows are your friend!
This photo (above) was taken facing a window. A good photo will have a much less drastic before/after because it was already pretty great to start with!
When you have good lighting, it's not only flattering, but you also have the opportunity for awesome colorful editing. When your photo is taken in bad lighting, it will probably be too yellow or grainy to make those pretty colors pop, no matter what app you're using.