Like many Mexicans, Manny wants to go to America because he seeks a better life for himself. He's a poor, homeless orphan, forced to live on the streets of Juarez. The United States and all the hope and opportunity it represents is just a short distance away, acting as a magnet for those like Manny desperate to escape lives of grinding poverty.
Aside from anything else, Manny feels that he's wasted in Juarez. He may only be a street kid, but he has a strong sense of pride and self-worth. He knows that he has the skill, the determination, and the street smarts to succeed in life. All he needs is to find the right place; and that place is America. The word on the street is that if you're willing to work hard, then you can make it in El Norte ("The North," i.e., the United States). And Manny has already shown in his short life that he has the capacity for serious graft. It's just that he's never been adequately rewarded for all his hard effort.