What specifically do you want to know? PA is not really different than any other state -
Generally this is how it will work - you are in 10th grade now - the summer after 11th grade or early (very early) the fall of your senior year you will apply to your congressional representative and the two US Senator's from PA.
The applications are much like a college application that will require transcripts to be sent from your high school and may require one or more essays.
The MOC's then decide who they will interview. Each congressional rep is a little different but the senators have interviewed in early November in the past.
You will usually find out if you are on a nomination slate sometime in Dec. Anything else??