The megapixel value is just a mathematical count of pixels. On its own, it doesn't say much about image quality. Lens quality and physical sensor size are the real main determinants.
Anyway, besides light and environment, look at composition, pose direction, and post processing.
Particularly with composition, be aware of your subject distance and focal length. Farther away with a longer focal length is generally more flattering and recommended for most portraits. But close-up with a short focal length gives a distortion effect that could be useful for certain costumes.
Some posing resources here:
Also make sure you have a good handle on metering, exposure, and focus with your camera (whichever controls are available). The better you have those nailed down, the less you have to worry about them and the more you can focus on creative decisions.
If you have a means for syncing off-camera flash, you can learn about using it here:
If not, avoid using flash as key light. I would stick with ambient and only flash maybe as fill.