If you really are planning to go to MetLife for FIFA (or coming to NYC/NJ this summer for any other reason):
Then lets talk about what you actually should do.
For the sake of this post, I'm going to assume that most people saying they plan to walk from Manhattan or Rutherford are just rage-baiting.
But! Here's the thing you need to worry about anyway:
Heat stroke. Heat exhaustion. Hyperthermia in general.
European tourists are famous for getting it in the United States. I do not want you to get that because you were waiting in the MetLife parking lot. If you're thinking "I can deal with it," then please. Don't. Unless you are also from a place that gets both hot and humid, don't.
If you are from a tropical or sub-tropical climate, then you can ignore me!
If you are from England or Germany in particular, this post is for you.
It is going to be at least 90ºF/32.2ºC for at least one game, probably more than one. There is a non-zero chance of it going above 100ºF/37.8ºC.
Humidity might also as high as 70%. This will make the temperatures harder to combat, because your sweat is not going to dry as fast. If you are not used to these temps and humidity combined, then do not underestimate them.
To compound the factors above, MetLife is surrounded by a sea of asphalt. It's something like 15.5km squared. The heat island effect caused by this is intense.
While not being able to park in the MetLife lot, there will be parking at American Dream. Drivers there are liable to be as reckless as those on the highway.
Surrounding the two of these is a marsh/swamp, which, you guessed it, increases the humidity, by a lot.
So what are we doing about it?
Bring water. Tourists often do not bring enough water. Plan for at least two bottles, or about a liter. If you aren't sure if that's too much/not enough, ask a local (hotel staff are reliable sources of information). You can get more drinks once you arrive at the stadium, but for the trip there and the way in, plan to bring at least a liter just in case. It is cheaper to buy at CVS/Walgreens/a bodega than to buy from whatever vendors might be in the parking lot.
(If you're hanging around the mall first, just one is fine. I don't know why you'd go to the mall, but maybe you're bored.)
If no locals are available, see if there are any Australians around. They will know.
Do not drink alcohol before the game if you don't have plans on how to get out of the heat as fast as possible. Probably don't drink towards the end of it, either. Alcohol speeds up the rate of dehydration, and thus the risk of heat-related illness.
Get a handheld fan. Could be motorized, could be just a regular one you wave in your face, but get one. If you're in Manhattan... listen, Penn Station and the train to MetLife may be air-conditioned, but the subway stations are not. Plan to be in muggy, stifling heat while you wait for a train to take you to Penn.
Do not walk more than you have to. If you plan on walking more than it takes to get from Point A to Point B, for whatever reason, bring an extra bottle of water.
Electrolytes. If you can get some sugar or salt in you with the water, that is helpful.
"I walk an hour on Sundays for fun! Americans are just not used to--" if you do not make that walk in these conditions regularly, you are not prepared. If there are heat advisory warnings going out for the metro area, heed them.
I'm not an expert in How To Survive A Heat Wave. I'm just a New Yorker who knows how I have to plan out my time if I'm doing something in the summer. I know how much tourists can struggle.
If someone is more experienced in this for NYC/NJ--outdoor staff, medical professionals, tour guides, whatever--feel free to add on.
Do not fuck with the cars once you leave Manhattan. I also hate them, trust me, but holy shit, do not risk your life like that. Do not try to walk on I-95, do not play chicken with the pavement princesses, do not assume that the semis can see you.
Do not fuck with the cops. NYPD is one of those notoriously violent departments, and this is not a safe time to be a foreigner in the US, even in progressive cities.