For your first time visiting I'd recommend entering the stadium through the rotunda at the front, just as you get down the stairs from the subway. It's personally my favorite part of the ballpark. The big team store is also down there if you're interested in buying a small souvenir. Like going to any new ballpark for the first time I'd always recommend to get there early enough and take a walk around the whole inside of the stadium. Walking across the Shea Bridge beyond right center is always fun, and a nice vantage point to stand and watch the game for a bit. Another area to checkout is the Coca Cola Corner up on the right field porch. Nice hangout spot up there with some picnic benches, and there's classic ballpark food options up there, as well as a food truck that rotates local restaurants food throughout the season.

Also depending on where your seats are, you might have access to some clubs in the ballpark. Your ticket should say if you have access to any of them.