If you are not sure wether or not you'll be doing more events that require a helmet, most do offer loaner helmets....however, loaner helmets are like loaner underwear.
For most motorsports events you'll need a helmet with an SA 2010 rating. It will be good until 2020. Watch out for SM ratings- those are motorcycle helmets, and usually not acceptable. Starting prices are $200-250. Full face is safer than open face. Get one, and get used to the look and feel. Speaking of feel, if at all possible, try a few on before buying. Helmet sizing can be as vague as shoe sizing. If your budget is a bit bigger, a nice option is a removable/washable liner. More expensive is not necessarily safer, just lighter, more optioned, or fancier. If you anticipate wearing it a lot in the summer, pick white over matte black.
Search "SA 2010 helmet" and have your money ready.