Before you get on a dirt dike for your first time you want to make sure you have the proper safety equipment like helmet, gloves, goggles, pants, etc. You do not want your first experience on a dirt bike to be a bad one when you could have prevented an injury by wearing proper safety equipment. The first thing you need to know before you get on a dirt bike is how to start it.
When you're straddling the dirt bike before you sit on it, put the kick stand up while hold the bike upright with both hands on the handlebars. Hold in the clutch and on your right side there will be a lever you push down with your foot. It may not work you first time as you are not sure the force you need to use to get it to turn over. However, after a few times it should turn over.
Before you get start riding it's important you know where each of the brakes are and how they work. There is a back break and a front brake which can later be used for tricks like wheelies. Your front brake is on the right handlebar which you squeeze, and your back brake is by your right foot where the gear shift is, but on the opposite side.
If you've ever drove a stick shift in a car, it will help when you're learning how to ride a dirt bike because you have the general concept of how the clutch works. After the bike is started and you're ready to start moving, on the left side there will be another lever you push down with your foot just once, this will put you in first gear. Be sure you have the clutch in this entire time otherwise the engine will shut off unless it's in neutral. Twisting the right handle will give it gas which you can hear by revving the engine. Give it a little gas and slowly let out on the clutch and you'll start moving.
Take note, that if you let out on the clutch too fast and do not have enough gas it will jump forward a little and then die, or if you give it a lot of gas and let the clutch out fast, hold on, because you're off to a very fast start. You'll eventually find the right amount of gas you need I order to get a smooth take off and a good way of doing this is to listen to your engine and how hard it's working. Now you know your way around the bike and can learn to ride it.