Since no cars are permitted on Fire Island during the summer months, your only (reasonable) option for getting to the island is to use the Fire Island ferries (or have a (friend with a) boat).
There are two companies who operate ferries between Long Island and Fire Island: the Sayville Ferry Service and Fire Island Ferries.
The Sayville Ferry Service runs ferries from Sayville (Long Island) to the eastern part of Fire Island which includes: Fire Island Pines, Cherry Grove, Sailors Haven, and Water Island. Fire Island Ferries operate ferry routes between Bay Shore (Long Island) and the western communities of: Kismet, Saltaire, Dunewood/Alantique, Fair Harbor, Ocean Beach, Seaview, and Ocean Bay Park.
Note: a third ferry service exists that takes passengers from Bay Shore to Point O' Woods, but it is private and usually filled with snub-nosed shit-heads who clearly can't spell.
The Fire Island ferry schedule confuses the hell out of most island visitors resulting in many people (many people) missing their desired ferry.
The first step in successfully making it on-time to your ferry is to determine what community on Fire Island you are leaving from. Although this may sound like a simple task, it becomes increasingly difficult relative to the amount of rocket fuel you've ingested.
Step two is to decide what day of the week it is since there is a different schedule for Monday-Thursday, Friday, Saturday, and Sunday.
Step three brings you to the part where you need to cross-check the current date (that's day and month) with the special notes on the ferry schedule under the current day (as some ferries only run before or after a certain date).
Finally you need to make sure you are looking at the correct side of the list of departure times. The left column describes the ferried leaving from Long Island, and the right indicates the ferries leaving from Fire Island (that's what you want).
RIDING THE FERRY (AND $)
You are not required to pay for the trip in advance (although it is an option – costs are equal). Once the trip is underway a crew member will come around to collect passage from everyone. The ferry is CASH ONLY, and a one-way ticket costs $9 (you also have the option of buying a roundtrip ticket for $17, but then must hold onto your return ticket). I do not know what happens if you have insufficient funds for the ride, but I am guessing they just take you back on the return trip.
Aboard the ferry you will ultimately need to make the all important decision as to where to sit. Personally, I always try to sit on the upper level (outside) and towards the back. If you choose to sit inside you will have the company of the roaring engine for the entirety of the trip.
Lastly, note that if you are bringing a large amount of luggage (read: coolers full of beer), you will likely be charged for doing so. Anything large (bikes or large wagons) will not be permitted on the ferries at any time, and you will have to send them over separately on the freight ferry.