Helmet what to buy and look out for

Hi, I finally going to collect my foldable bike later. OK, next is a helmet. I have never wear one before. What are the basic thing to look out for? Free size?

as long as it fix well on ur head and u like it
most come in different sizes

I went to bike shop this afternoon. They carry the Met brand helmet. One was retailed for $65 and another over a $100.

The $65 comes attached with a 'tail' blinking light at the back of the helmet. Is that a good buy for normal road riding?

i cant comment if its a good buy but its good to have blinking light as a awareness to other road users