as a complete newbie, ive been a bit unsure as to how to go about buying a helmet that fits rather than a helmet someone at a store is going to tell me fits. any other tips would be helpful.
If you ever find someone that actually knows how to fit helmets, post'em up so we can all give them our business in the future.
Bumblebee's method based on having been through a proper fitting of a flight helmet way back when and multiple webboard/in-person discussions on the matter with other riders:
First off, pricetags and color - don't even look. $10 or $1500, try them all on. Fit first, color second, price third - IN THAT ORDER. (Yes I put a screaming pink helmet on one day for half an hour that had cute little teddy bears and flowers all over it. I eventually went with screaming yellow and another brand however I'm still wondering if the image of a guy with a cute pink helmet would help cagers spot me better or not. I have no shame because the ground is a brutal thing to have slam into you)
If you've never worn a helmet and get one that fits comfortably and think it's right for you, it's almost certainly the wrong fit for you.
Grab one that is in the general size range you're looking for.
Put it on your noggin.
You're looking for one that contacts your head equally all the way around your skull. There are different head shapes and you'll have to experiment around until you find the brand/model that fits your specific head shape.
Internal shape and fit: You're looking for quite snug that doesn't squish your brains out. There's a difference between snug and pressure, you're looking for symmetric snug all the way around. (If you're having a hard time determining skull fit, pull the cheekpads so you can feel the skull padding better without the distracting pressure of the cheek pads)
Once you find one that's the right shape, try the size up and size down. Which fits best? If it's hard to get on and squishes your ears getting it on/off, you're in the ballpark.
Now things start getting uncomfortable a bit.
Hold your head still and try to turn the helmet side to side. Your head should go with the helmet. Now tilt it up and down. Again, your head should go with it. There is always some helmet movement independent of your head. You're looking for skin movement feelings. (Put your hands securely on your bare head and move your hands, that's what the helmet should do to your skin) IIRC I can turn my helmet side/side about half an inch and up/down less than an inch.
Now secure the chin strap that's been loose up to this point.
Go through the twisting/pitching routine again. It should be a little more secure.
Reach over the top and grab the back bottom and try to roll the helmet off over your head. Pull pretty hard and try to get the helmet off. It'll lift up a bit however you should not be able to get the helmet off.
Push up on the bottom of the chin bar and try to get it off that way. Same procedure as before.
Now put your hand on the chinbar and push hard. You should not be able to touch your nose on the chin bar. (If everything else fits and you can force the chin bar into your nose, try larger cheek pads)
Check for good visibility, not overly large blind spots with the chin bar, etc etc.
Once that's done and it seems like the right one, go window shopping around the store for half an hour. This is to let the helmet form to your head to see if it violates any of the above procedure steps. It'll also show you any pressure points you missed in the initial fitting. (I had one that I was ready to buy however after 15 minutes it felt like someone was trying to push a pool table ball through my forehead with a hydraulic press)
IMNSHO: Full face is mandatory becuse nothing less is adequate protection. Q.E.D. Flip ups, while nice don't have the tight cheek pads keeping the chin bar away from your nose. When the ground jumps up and slams into your face, it'll try to ram the chinbar into your nose. Those pads help keep the chinbar away from your nose and even then you're likely to get nose punched anyway.
I think that's all there is to it. If I missed anything or got anything wrong, feel free to correct me. This is about safety, nothing more, nothing less.
If all is ok, then shop for color and pricing. Pay up and go riding.
As with all crash gear, I sincerely hope you wasted your hard earned money on something you'll never actually need for the intended purpose.