Protective gear is as important as the motor cross bike. The gear protects you from flying dirt, debris and the occasional spill that happens when you ride motorcross bikes.
First off, get the best helmet you can afford. It may very well save your life. Don't borrow a helmet, don't use an old helmet. Remember, helmets are good for one impact only and you have no way of knowing that helmet has been impacted or dropped from a height that would compromise the interior. Best bet: get a yourself a brand new full face helmet that will protect your face and chin.
Goggles are oh so important. A lot of dirt gets kicked up when you're riding on a motorcross track and if a tiny bit of dirt enters your eye, it's going to hit with the force of a bullet and can cause damage. This is easily prevented by wearing a good fitting pair of goggles. Make sure they fit your face well and don't slip.
A pair of gloves for good grip is a must. Motorcross is not only a hot sport, it's a HOT sport. You can get very sweaty and sweaty hands mean less grip. Get a good pair of gloves with rubberized palms and rubber or silicone fingertips.
Chest protectors are very important. You're traveling on rolling dirt tracks with other riders at fairly good rates of speed and accidents sometimes happen. A chest protector can save you from serious injury by absorbing the impact of a fall. A kidney belt is a great addition to your body armor as it minimizes the jarring and vibrations that your kidneys feel when your ride motorcross. Pads are very important, get them for your knees, elbows, and hips.
Last but not least, get yourself a tall pair of boots. The boots should have reenforced shins and be made with sturdy shin lines and toes. Remember, when it comes to your safety, there is no such thing as too much safety gear.