*sigh* Confusing.
Ok. You're on Mac Snow Leopard. Presumably your parents or other relatives own the computer, have created the admin account, and given you a standard account. You wish to use the terminal to go around the back door and deactivate their admin permissions, and give them to yourself. Correct?
Firstly this isn't a smart idea. Whoever you take admin permissions away from will most likely not appreciate it. For this reason, I'm tempted to not tell you how to do it unless you state sufficient grounds (unless it's a private matter, then say so).
However, why not just do the simple thing and log in under the admin account and do it through System Preferences...?
EDIT: One other suggestion, off topic. Might I suggest from now on if you have several questions, to put them into one thread? You've created 5 threads in the last week, having them all in one thread, possibly entitled "Many Questions" is much better for the forums (makes them look tidier), and saves you the risk of getting a warning for spam. Unless of course you're doing it for the post count, which is just as bad.