White balance and 18% gray

No no - it doesn't mean that. It means that if you put a gray card (propped up, for example) on the set and photograph it, the camera will find it as 18% gray (instead of the white background) thereby recording the white as white. What's happening is your camera thinks you WANT the white to be gray. You need to feed the meter something almost PERFECTLY 18% gray - that's why they make the cards. . .if they didn't, photographers everywhere would be in a fix.

So here's what you do:

Set up your white backdrop, and put the camera-store purchased gray card (they'll know) where your main subject is going to be.

Compose, focus, and shoot. That should be sufficient for a good start.

(If the background still isn't as white as you feel it should be, fill the frame with the gray card and do a test shoot again).

The histogram should show a spike for the gray, and the white should look pretty much white.

From there you can lock the white balance or set the white balance (read your camera manual on how to do this) and/or the exposure.

Later, in your image software (which one are you using? Photoshop?), go to the "Levels" selection and select the gray eyedropper. Sample the gray card in the photograph with that eyedropper, and it'll adjust to the mid tone, and the whites will become white. You can learn to apply that adjustment to all the images in the shoot automatically, but I'll leave that for you to research (lots of typing!).

Is this making sense?