Let's deal with the pain first! You'll need to get the bike set up properly for you to achieve a good riding position. Your local bike shop or licensed bike coach can help you with this. I just posted the basics for setting up a road bike in another thread on this site titled "bike fit question" if you want to try it yourself. Setting up a mountain or hybrid bike is different. Good, well-padded bike shorts are really important too!
Then you just need to ride for a while. Don't worry about formalising training for the first few weeks; just get out on the bike and enjoy the rides. Aim to increase your distance gently after a couple of weeks. Get comfortable with your bike's handling and ride characteristics.
Take at least one day off the bike each week.
After a month or so you should be ready to get into a more formal training program. I highly recommend the Chris Carmichael / Lance Armstrong Performance Program book which is really good reading and offers great general advice and some sample programs too.
I also recommend the Carmiachael/Armstrong book. Many people complain the first couple of weeks in the program is too easy. There's a reason for that - it's to build a good base for the last three weeks which are much harder. So it's important to stick with it even if it does seem a doddle for a while.
I wish I were starting out at 18. I remember those days when I could burn off a pound of fat in a couple of hours. I am 41 and haven't had a bike since my homeade BMX 2-speed kick back Schwinn was stolen from me the summer after 6th grade. I loved that bike and the fact that I bought it, stripped it, had it custom painted and put on the moto X handle bars and knobby tires made it real painful to lose... Anyway, fast forward 28 summers and I got the bug real bad. I just bought a used road bike and I feel like a little kid getting his first bike. Remember what that was like getting your first bike? When I was 12 I would ride all over Las Vegas on those hot summer days picking up odd jobs to keep plenty of Slurpee's in supply. Well, I've been riding for about 6 weeks with remarkable results. I am going through a huge learning curve, but the more I ride the better I get. Got the Carmichael book and have found that even at my age the sky is the limit. No Slurpees, though.