Helmet
The key is finding one that fits you. To find the right size us a tape measure to measure around the largest part of your head - about 2-3cms above your eyebrows.
A good fitting helmet will be snug not tight, and sit level on your head, with the front edge no more than 2-3cms above your eyebrows.
Push the helmet side to side and back and front, if it moves significantly in any direction, use the sizing pads to snug it up, and tighten the chinstrap.