If your base is 3-4 miles, and you intend to run a marathon in 12 weeks, you are very likely to either hurt yourself, or fail to succeed.
While it is possible you can do it, you are starting from a very low volume point. I would suggest considering training more to build up a stronger base before considering a marathon.
Usually the plan is 10% improvement a week, on a 4-5 week schedule of
increase
increase
increase
increase
down week
increase
increase
increase
increase
down week
and so on. That is you slowly increase volume each week for 3-4 weeks, then drop back for a rest week for recovery, but not all the way back to nothing, and thus you start back building from a higher point each time.
Your problem is you are starting at 4 miles, and you want to get to the point where you run 18-20 miles in training in a single run. Doing this in 12 weeks will be problematic, and will require some big jumps in distance which your body may not be able to handle (thus the concern about injury).
Of course your mileage may vary.
The more I think about this, the more I would suggest if you are determined, to pick a marathon a bit later in the year. There are races almost every single weekend of the year in North America. Even in the fall season, they start in September and run till December for big name races. (Toronto has an early September one, Chicago/Cincy/DC are in October. November is NYC/Philly, and then there are some in December as well, even Jan for Disney and when is Vegas again?)
This is really risky. If you do go ahead, be prepared for injuries, and be willing to bail on the goal race, as it will just hurt you if you push through the injuries.
As for the rest of the questions:
Gear: Get the proper shoes. Find a reputable running store (Not Footlocker, or big name shoe chain. You want a running store. Running Room in Canada, JackRabbits in NYC, or some other style store). If they do not watch you run on a treadmill, or watch you walk a straight line, or make you balance on one foot to see which way your foot falls, walk away. (All those are valid methods).
Get the proper clothes: It is summer, it is hot in most of the northern hemisphere. Get wicking clothes, they work, and make a huge difference.
I like a water bottle, on a bottle carrier. Others prefer the silly smaller bottles on Fuel Belts, whatever. On the long summer runs, you need to carry (Or stash ahead of time on your route) water and drink it.
Sites like Mapmyrun.com are excellent at integrating with the GPS in your phone to track where you ran, show you a map, and store for later review. Or else simply map it later on Google Map or GMaps Pedometer. Or else get a GPS watch and upload the save sets to some mapping site.
Otherwise this is all about teaching your legs to run (muscles) and your heart/lungs to handle the demand (Cardio). There are no short cuts around those two things. The best way to train to run long distance, is to run long distance. TANSTAAFL. Alas.