You don't need a lot of training to race Motocross. There aren't a lot of necessary requirements - however, commonsense is a huge factor when you want to race motocross. Knowing the basic techniques of when the accelerate and when to brake are great for the beginner to learn basic bike control. But before you even get that far, look closely at your motorcross bike.
First of all, make sure that your bike is the proper size. Your motocross bike should be the right size for your body to insure proper handling and safety when your ride. Additionally, make sure you have a helmet. It should be DOT approved and not used. Helmets are good for one impact only, if the helmet you're using has been dropped from a height or used previously, it is suspect and shouldn't be used. Safety is paramount. Invest in good pants, chest protector, knee pads, elbow pads, and hip pads. One spill on your bike will pay for the pads.
Next, when you're safely suited up and ready to roll. Simply ride over the jumps and gently increase your speed until you're comfortable with them. You'll gain skill and confidence the more you practice. When you're able to keep steady with the other motocross riders when you're doing your practice rides, you're ready to race.
Expect a few spills, expect to make some mistakes when you start out. No one is born knowing exactly what to do and it just takes practice. Motocross is a fun sport that anyone can get into and if you practice your riding skills and always make sure you are wearing the proper safety gear, you can have a blast competing on the track.