Todd and Andrew did a good job explaining the game of baseball, so I'll just chip in my two cents
The Giants are one of the better teams in the National League, and the Mets, moreso due to many of their star players being injured this year, are not.
The Mets best player right now is probably Jeromy Burnitz, who is an outfielder.
That said, watch for flying balls / bats. Depending on where you are sitting, when the ball is hit foul, it usually ends up in the stands.
and that said... have fun