In order to complete a marathon, you have to find a way to run with pain. It's going to hurt, no matter how well you train. Even when you slow down or walk briefly at water stops to grab a drink, your feet will not catch a break until after the race.
That being said, you're smart to train for a 5k and build on that for about 6 months. If you haven't already found a training plan, get one. It will take the guesswork out, and you'll see what distances and what types of runs to do on which days.
As far as shoes, go to a reputable running shoe store and have a rep analyze your gait. They will recommend a shoe that's right for you. If you can afford two pairs, buy two, because your first pair will probably wear out leading up to your marathon. (General rule of thumb is 250-300 miles per pair of running shoes before they start to break down.)
Depending on what you wear for a shirt, your nipples might chafe. Not fun. If that happens, put Band Aids over them before you get dressed and they won't chafe.
If you wear cotton underwear under shorts to run, again you may chafe because cotton will absorb moisture, and your inner thighs will come into contact with the cotton while you run, which will feel like razor blades slicing into your skin, especially when your sweat finds the chafed skin. Wear running shorts with a built in liner and if you need more support than that, wear something like an Under Armour brief.
Stretch before and after every run. Don't be afraid of taking Ibuprofen afterward if you're achy. Hydrate with an electrolyte replacement after your run. Make sure you eat well, and get good rest throughout your training but especially leading up to a marathon.