If there's anything that sponsors want to see, it would be that they are getting something out of this. They are looking for a return on their investment, what they will benefit if they were to...
it depends on what you want to get sponsored for. Make a video of you doing whatever you can do and send it to a company that specializes in your sport, it helps if you put in a bit of background...
By asking. Write letters, emails and make phone calls. If you think you have a good company in mind to sponsor you, ask them, all they can do is tell you no. And then you are just where you started....