To work properly, a bike helmet must fit correctly and be worn properly.
Sizing your helmet
In order to determine the size you need, you need to measure the circumference of your head.
Simply wrap a measuring tape around your head, being sure to locate the tape just above your ears and keeping the tape level front to back.
Once you've determined the proper size, you can check to make sure the helmet fits correctly.
If the helmet you own now doesn't fit on your head with just a little space between your head and the headband when the fit system is loose, you should replace it with one that does.
You should always wear the helmet positioned low enough in the front to protect your forehead, but not so low as to hinder your vision.