If you value your brain, a bike helmet is an essential safety measure.
Things to Consider for Buying a Cycle Helmet:
Fit is all important. A bicycle helmet can’t protect your head if it doesn’t fit on it. Research before you buy to make sure the bike helmet you get has well functioning clasps, so it does not fall off your head.
The brighter the colour the safer you are. Unless you really really want to avoid being compared to a traffic cone, it is wise to get a bright helmet, so you can be seen.
This is a balancing exercise between more padding, to say safe, and not cooking your head. Try on the helmet if you can.
Most helmets weigh less than 300 grams. However, you may want to double check, a heavy head not only makes you less comfortable, but also less aware. The less aware you are, the less safe you are.
Style-wise, there are lots of bike helmet types out there. However, make sure style does not overtake form. This being said, the type of helmet you get should be related to what you like the look off, and what you think will suit you.