You don't have to wear a helmet by law and there's a big debate about whether or not it's a good idea, so ask your parents what they want you to do.
If you do get a helmet, make sure it fits you properly, doesn't block your eyes or ears, and is secure on your head.
It should also have a recognised Safety Standard number like EN1078.
Also, don't wear clothes which are too baggy as they could get tangled in your chain or wheels.
And wear bright clothes in the day and reflective clothes or strips at night - you're a lot harder to see than a car!