Choosing a helmet
Well, the first thing is to make sure it's a DOT approved helmet (if you're in the USA). All sold in stores will be, I think, but on the Internet you can't be sure. I've even heard of some made-in-China helmets that have DOT stickers on them, but have really never been tested. Buyer beware. Better than DOT approval is the more rigid Snell Memorial Foundation approval.
Next you have to decide on a level of coverage: half helmet, 3/4 helmet, or full face. If full face, you have to decide on a one piece or modular (flip-up chin guard). I prefer a modular for the extra protection over a 3/4, but some find them too confining. Half helmets are only slighter better than no helmet. I don't recommend them.
Next, you have to find a size that fits; snug for safety, but not too tight for comfort.
Really, it's a pretty personal matter. I suggest going to a large dealer and looking at several styles and models. If they have a helmet specialist on site he or she can help you get the one most right for you. Even if you later decide to buy on the Internet, shopping at a dealer where you can see them up close and personal will be helpful. (I would buy some small item from the salesperson if one helps you. Gloves or a Throttle Rocker or something, and tell him or her that you're just thinking about a new helmet at this time).