Ok, I'll add in here. :) This probally applies a little more when you can drive said girlfriend to dates, and bring her home to meet momma. But Always treat your mother with respect. Girls are taught that the way a boy treats his mom is the way he will treat his wife.
I remember when it seemed important to have a boyfriend at age 12. so I can imagine the emotions ring true for a boy to have a girlfriend.
Find someone who shares similar intrests then you will have something to talk about.
Know mom and dad's phone rules. They may not be what you would expect. and know ahead of time if using cell phone minutes, etc. is acceptable. The punishment for messing up is not worth it. Phone bills are expensive if misused.
Other than that try to know some current events, pop culture, the latest movies. Make yourself informed and knowledgable. But listen to her opinions.