Championship Points are earned by placing high in the Tournament Standings. Play! Points are earned by simply attending events and are awarded regardless of finish. So play in enough events and you will be eligible to play in the National Championships.
The 2013 Pokémon TCG United States National Championship will probably take place in Indianapolis, Indiana again late June going into July.
Here is the official Pokémon TCG Agenda for Nationals once they share some info.