What is a cheap, but good homemade self-defence weapon?

Living in New York is scary. People get mugged every day. I already have a pocket knife, but it is really dull, and that's all I could afford. My parents don't have enough money for a gun, and I need to protect them. I am 17 years old. I am using my friends internet. My question is, what is a good homemade self-defence weapon? Don't say bow and arrow, because that would be weird to see someone walkin' around NY with a bow. If you have a video please send it.