Hi, I don't know where you got your bike but a good place to get a helmet is at a local bike shop if you have one near you. They can help you pick the right size and type. The other place that comes to mind is Dick's Sporting Goods (or any sporting goods store) if you have one of those.
I have really gotten into the biking over the past few years and love it. It's great exercise and it feels great to get outside to exercise. We have a lot of "rail trails" where I live which are paved bike paths that used to be RR tracks. It's great because you don't have to worry about the crazy drivers...
Good luck with your cycling!