How do i start racing motocross?

Ok, a "pimped out 125" doesn't make a difference in racing. You'll be better of with the stock bike racing. You don't need any license, but you need to look up the closest MX track near you. If they don't have a website, go talk to them in person. You'll need to register with them and they'll go over the rules with you. You don't need any license. Talk to them and you'll figure it out. Most important, you need to know technique. Learn the MX attack position if you don't already know it and pinch your knees when you need to. You don't want to race out there with people thinking you're a goon. It doesn't create good relationships with people out at the track. Don't try to show off. Don't rev your bike in the pits unless you're warming it up. Don't go any faster than 3 mph through the pits. Last, don't tell people about your "pimped out KX125" and what it has on it.

Go to a race track, look, listen and ask questions politely. don't try to grandstand and appear as anything but someone wanting to start to race. Most guys will be able to give you good sound advise.