Start with certification
Look for a real safety certification such as ECE or Snell. DOT-only helmets can vary widely in quality, and novelty helmets should be avoided completely.
Fit beats brand loyalty
A good helmet should feel evenly snug around your whole head without sharp pressure points. Different helmets fit different head shapes, so a popular helmet can still be wrong for you.
If the helmet shifts easily when you shake your head, it is too loose. If it creates pain after a short try-on, it may be the wrong internal shape.
Full-face first
For most new riders, a full-face helmet is the safest default. It protects the chin and face, blocks wind and debris, and reduces distractions compared with more exposed styles.
Do not buy a used helmet unless you fully trust its history. Damage is not always visible, and helmets are designed to absorb impact once.