The first thing you need to do is find a lawyer who specializes in immigration cases. You will not be able to get a good attorney if he or she does not have experience with this type of case, so it should be one of the first things you look for when searching for an attorney.
After that, select an attorney from those who specialize in lost passport cases. The reason being is that they are very experienced at dealing with these types of cases and can explain what needs to happen next as quickly as possible. They will know exactly how best to go about getting your passport back and helping you deal with all the other problems associated with missing passports (such as replacing stolen documents).