There are some good ideas on this forum section if you are prepared to weed through but if you are just looking to enjoy and finish the distance, and not break any records, it can be kept relatively simple.
3 months is plenty of time for someone in relativley good health and not on the tail end of the masters category to get in shape. I would make sure that I ride at least 4 times a week with one long ride (3+hrs) being introduced a couple weeks after the start of the plan and every couple weeks increase the number of hours/miles a little bit. In the early weeks it's a good idea to keep the intensity moderate.
Not sure what your schedule is like but if you are riding 4 times a week (5 times is better), with 3 of the rides being around 25-35miles, then the long ride could step up something like the below. If you are comfortable riding more, even better. Each 4th week gives a break to recharge. Starting at about week5, during at least one of the weekly rides, I would add a couple of long intervals of about 15-20 minutes at around 80% of your maxHR, with about 10 minutes rest between each. If you don't have a HR monitor, this would be about a 7 on a scale of 1-10 (short sentences possible). Weeks 8,9,10 I would try and add some hills if you haven't done so already, so you could spend at least periods of around 5 minutes huffing pretty hard at an intensity where talking is difficult (and only very short sentences or single words are possible). This kind of intensity goes a long way and not much is needed. If you are new to athletics in general, and since you are not training for a race, they could even be skipped altogether.
Length of the "long" ride:
Week01-same as the other rides (30-35miles)
Week02-same as the other rides
Week03-40miles
Week04-50miles
Week05-40miles (add 2x15mins@80%maxHR)
Week06-50miles (add 2x15mins@80%maxHR)
Week07-60miles (add 2x20mins@80%maxHR)
Week08-50miles (add 2x20mins@80%maxHR)
Week09-60miles (add 2x20mins@80%maxHR, or replace with the shorter hard efforts/hills)
Week10-70miles (add 2x20mins@80%maxHR, or replace with the shorter hard efforts/hills)
Week11-50miles (still ride, but take this week nice and easy so you are fresh for the big day)
Week12-event
This is just off the top of my head and a plan should be tailored to the specific rider and the course (flat vs. hilly will make a difference). People are different, some will have dfifferent ideas of how to prepare but this would work for me if I wasn't in great shape to start with. Personally I wouldn't need much more than regular riding for a couple months before and at least a 70mile ride a couple weeks out before the big day. You may be different. Plenty of rest, good nutrition, and not doing anything that feels harmful to your body training-wise are a given. At the end of the day all the interval stuff can be skipped, it's just an easy way to keep track of slowly building up intensity without adding too much too soon, and to increase cruising speed. The main key to your fitness is to ride consistently and build up mileage/intensity slowly.
Make sure to not start the event too hard - the last 10 miles are going to be tough. Most first time century riders who have been training start with a bang because they are in the best shape of their lives and excited to get going. Drink a lot of fluids, make sure you have enough sodium, and make sure to eat during the ride. Make sure to have a patch kit, and learn how to change a tire or ride with someone who does.
Good luck - I hope you have fun.
Edit: Btw, a big bookstore should have some good stuff available. Joe Friels Training Bible is a good reference too. If you are really short on time (i.e. 6 hours a week) maybe consider Chris Carmichael's Time Crunch Plan for centuries, but that is very interval driven.
