how should INeed to obtain a green card but have a felony?

You have a major problem. Even if you did not have the conviction, sounds like the arrest involves a situation where an officer may have "reason to believe" that you are engaged or were at some point engaged in the illicit trafficking of a controlled substance. The idea here is that the immigration law provides an avenue by which an application for admission to the US (i.e. through a petition filed by a member of your family) can ultimately be denied based on this "reason to believe" charge without the existence of a conviction. So the conviction is not the issue as much as what the police report, arrest affidavit and other competent and probative evidence relating to your arrest may establish. So instead of going from lawyer to lawyer, each telling you how wonderful they are and how much they can do if you just pay them first, I would recommend finding someone who is experienced and competent enough to figure this out for you ahead of time. This is a tough case with which to have success. There may be a way out of it if you have the right attorney and if the truth is that you were not involved in what the evidence and the fact of your conviction may say. Choose wisely and good luck.