Motocross in the United States is an extreme sport with extreme costs and extreme dangers. To participate in this sport you must be physically fit. The learning curve is broken bones. It is not "if" you get hurt, it is "when" you get hurt. You will need proper gear, proper equipment, means of transportation and good health insurance. Some kinds of insurances do not cover motorized accidents, read the fine print and make sure you are covered. AMA provides an insurance, Armour Insurance, that will cover you during AMA organized races.
Some of the motocross gear need: Pants, jersey, sox, helmets, leatt brace, knee braces, kidney belts, boots, gloves, goggles, chest protector and more. If you make podium there are sponsorship programs available at some of your local sports dealers. They can help tremendously, worth checking into. Not only is this sport extreme, it is an extremely expensive sport, take all the help you can get.
There are all types of bike sizes. Your local sports dealer can help determine the best size for your according to your age, weight and experience. New or used dirt bikes and cost up to $8000.00.
When you get your bike you will need something to haul it to the track with. Consider mileage to the tracks you will be racing at. You can go with a small open or enclosed trailer, a fun hauler, large enclosed trailer, load it up on the back of a truck or I have seen small bikes in the trunk of the car.
If this is something you would like to see, get out on the weekend and find your local MX track and do some investigating. Make sure you dress appropriately. Most tracks are very windy. In the Spring and Fall it can be cold. Be prepared for rain, just like any other outdoor sport, the race will continue.