If you are going to do downhill, full body armour is vital. Other than that basically three things, helmet (most important), gloves and shades. The glasses are not for the sun, rather for the small stones that are flung into your face and eyes by the rider in front of you on a stretch of single track.

And, maybe a good pair of cycling shorts, not really for protection from falls but more for protection of all the "sensitive" parts that will chafe on the seat.