I think IM training can be a little individual, what works for some in terms of volume and intensity may not work for others. Most important is getting a decent number of hours sleep at night - this is when your body repairs itself, and where injury risks can be reduced, and adjusting your diet. Train more, eat more.
The other thing is not to launch suddenly into 18 hours a week of training thinking it'll help - it won't! You'll more than likely end up exhausted and injured.
One of the best things I did was to join a local road cycling club. With them I do a long ride of 4-5 hours every weekend. The routes always vary so it doesn't get mundane, there's safety in numbers, it can help with motivation - you'll do the miles because everyone else is! I got lots of tips etc from people that had been riding for years. Other than this I was, well still am, doing rides of approx 2 hours during the week, plus brick sessions of a ride follwed by a 30 min or so run.
You could also consider a running club. I found the local tri club wasn't very helpful as it was geared purely to distances sprint to Olympic. But road cycling clubs and running clubs have much more scope for getting the serious miles in within a group environment.
Spin classes - it depends if the instructor is a cyclist or a taught spin instructor. I have tried both, and in my experience, unless their cyclists the workouts bare very little resemblence to what takes place on the road, amd what you need to be able to do. The positions they ask you to 'hold' on the spin bikes are also pretty unhelpful. Personally I believe 'bike is best'. You won't be on a spin bike on the day, so why waste time training on one? I have a turbo trainer for when the weather is really bad, or I only have an hour to spare and need a time efficient way to train. Spin classes are also really short. You'll be on the bike for a number of hours (maybe 6-7) on the day, and all the muscles in your body need time to adjust to the position they'll be expected to hold on your bike.
Body pump/circuits - again, I wouldn't waste precious training time! That's about 2-3 hours swim or running time down the drain! Plus those classes are only an hour or so at a time. You'll need way more stamina than that on the day!
For the first timer I think taking time to do the mileage, and building it up gradually in a sensible way, is the key. Take care of the minutes and the miles to take care of themselves.
I entered various road running races before the day, and didn't use an MP3 on any of them. And of course triathlons you can't either. That way I got in the mindset of only wearing one during training (I never use one on the bike at any time, training or racing, I consider it to be too dangerous). But really it's a non negotiable factor, you can't have one, so why worry about it! Everyone is in exactly the same position. You'd also lose all the atmosphere of the day if you were shut off in your own MP3 world. And for us age groupers that's one of the reasons for doing it. But if your worried about not being able to survive without one, then the only way to get over that is to train without one, that way on the day it's normal.
Bottom line is - you need to swim, bike and run. To complete this distance, from nothing, in less than a year, they are the only things you need to think about doing.
All this is only my humble opinion, but it got me around. I totally acknowledge there are many ways to do the deed! And Bella along with everyone else on here is far superior when it comes to giving out advice in triathlon!