Ninjas spend their entire life training, so you cannot just "become" a ninja, but this page contains some ways you can get started. Being a ninja doesn't mean you have to go around killing people, I mean, you can if you want, but there are other things you can do if killing isn't your favorite passtime.
First of all, you will need clothes that are low in weight, dark, and do not make noise when you move. You might want pants with multiple pockets if you are going to get serious. Make sure they are comfortable and you are able to be flexible while wearing them. Make sure the pants don't rip when you bend over.
Your shoes are very crucial. They should be comfortable and tight. Also, they will need good grip so you can climb easier. When you lift your feet of the ground the should not make noise, new shoes tend to squeak. As with all ninja apparel, they should be dark.
Ninjas tend to travel light. You don't need much unless the task you are doing specifically needs it. Usually you should not have much more than a small flashlight. Carrying smoke bombs and such can be a dangerous thing, because if for some reason you are caught, you will be in more trouble than is necessary. Try to carry as little illegal items as possible.
Before you set out on your mission. Do a couple of stretches to make your muscles more efficient. Stretching also makes you more flexible. Try to clear your mind, when you are being a ninja, you must use your full concentration. Basically, try to have as little things on your mind as possible, and example would be eating if you are hungry.
After you decide what you are going to do, plan it out. Go to the scene and make sure you know your way around. Plan a couple escape routes, you must be prepared for an emergency. Also, you might need to think of an excuse to be in the area if someone stops you and you did not get a chance to hide. Also, having a getaway car wouldn't hurt, you want to be in public for as little time as possible.
These are things I have learned on my many adventures, if you have any questions or comments, feel free to email them to me.