How can I obtain lawful permanent residency if I am already in the U.S.?

First of all, you have to have a basis to apply for lawful permanent residency. If you don't have a basis to apply then you don't have any options. Since this site only deals with family-based immigration issues I will only discuss those options. There may be other paths to lawful permanent residency through employment. So, in order to stay in the U.S. and obtain residency here you would have to be an immediate relative of a U.S. Citizen (spouse, child (unmarried and under 21), or parent. You must also have entered the country legally. If you don't meet these requirements you can not apply for lawful permanent residency while you are in the U.S. If you have a relative that is a U.S. Citizen they may sill be able to apply for you but you would have to return to your country and wait for the visa to be approved.