Everyone gets to vote for the candidate that they want in a primary, but the delegates you elect are the ones who get to actually vote for a presidential candidate in the national conventions. They also get to vote on their parties’ platforms, so even if their candidate isn’t nominated for president, a delegation can still influence the party. On the state and local level, Ron Paul Republicans have made a significant impact already.
In Pennsylvania, you directly elect the delegates you want, so you need to know where they stand. Go here to find out your congressional district and see which delegates are for Paul, and vote for them on April 22. You can also print out sample ballots for each district to bring with to the polling station or to hand out to other people.