I arrive around 5-530 PM. I usually go to GAME ON, a local bar and grab a pizza and beer. Going to a RedSox game isnt simply a baseball game, its an event. The buzz around the park before games is what gets me most excited to go to Fenway.
There is very little room to walk around, its very cramped. If you want to walk around.....do it before the game starts. You can pretty much see any area you want except the Green Monster seating area, and the VIP areas.
The best food outside the stadium is the sausage guy sausages. And yes, there is a difference between the different sausage carts. Sausage guy makes his own sausage, while most others just pick up a pack of sweet italians from their local supermarket.
The best food inside the stadium is the Fenway Frank. It is a Kayem hot dog that is steamed/or grilles and has a hint of smoky/spiciness. Get it with spicy mustard only. Do not get them from concession stands (they grill them on rollers), rather....get them from a vendor in the stands (those ones are steamed). Vendor dogs are more authentic.
As far as drink, basically anywhere you go......you will get a watered down draft pilsner beer. Make sure you get 2 at a time (dont want to look like a rookie).
Dont worry about being polite, people understand that the place is cramped. Most people are more than accommodating to stand up and let you pass to do whatever you have to do (although I try to wait in between innings).
Clothing, if its 60 or up, I recommend shorts and a T shirt......and bring a pullover sweatshirt or light jacket. Less than 60, dress to be a little colder.