Be prepared to try on LOTS of helmets before you find one that fits. Unless you're racing, you'll probably want to avoid the ones that have the shell covering your ear. Look, instead for soft and even removable ear pads. If you tend to get hot, look for lots of vents, or adjustable venting systems. The reverse is true, of course, if you're always cold. If you have a very small or short head, try a junior helmet. You'll also find that each manufacturer has a different 'fit shape'....some are narrower, some are shorter front-to-back, some will sit higher on your head.
The helmet should fit down to just over your eyebrows and should cup the back of your head firmly. If you think you found one that fits, put it on your head, use both hand and try to twist it side to side. It should not move. Then try to roll it off the front or back of your head. It should pull your brow and your hair but not come off. Lastly, bend over and shake your head vigorously. If it doesn't fall off and nothing hurts yet, buy it. :D
Then, as suggested above, put your goggles on with your helmet. They should still not gap around your face and there should not be a gap between the top of the goggle frame and the bottom of the helmet. You'll also want to make sure the strap on your goggles is long enough that the goggles don't either pull away from your face or pull too tight into your face.