[barsikjal]

I don't know what you should get or what your price range is but I wholeheartedly recommend a helmet.

It only takes one pop on the noggin to kill you or leave you a vegetable. You could hit a root or a stump or slip off a rock or anything like that riding on a trail and fall into a tree or onto a rock or anything. Even a small strike could leave you concussed, bleeding, etc. When I used to ride my mountain bike, I made good use of two helmets.

If you want the best ventilation in the biz, Catlike is probably the way go to but be prepared to pay. The Kompact and Whisper are crazy but they're more road-geared. Aside from that, you cannot go wrong with any of the big brands like Specialized, Giro, Bell, etc. Get yourself a helmet that's molded in the shell. It'll be lighter and have larger vents while retaining its overall strength.

More important than anything is correct fit and proper adjustment. If you've never done it before, get it done for you at a shop. If you wear a helmet incorrectly it could do nothing to help you or even cause more damage in an impact.