Having only competed in a handful of triathlons, I'm still new to the multisport lifestyle. I've definitely made mistakes along the way and wanted to share some of the things I've learned with anyone thinking of doing their first triathlon.
Sign up for a triathlon. This is usually the hardest part for most people. For me, it was the biggest motivation to actually start training and working towards my goals.
Don't be intimidated. Yes, it's new and scary and there are lots of moving parts, but when you break it down to the basic building blocks, it's just "take a swim, change clothes, ride a bike, change shoes, go for a walk/run." The distance and speed are really up to you.
Don't use lack of equipment as an excuse not to start training. If you have a swimsuit, can borrow a friend's bike (road bike, mountain bike, or even beach cruiser), and have a pair of running shoes, then you have everything you need to train for a triathlon.
Find a program that fits your schedule. If you can only work out 3 times a week, there's a program for you. If you want to work out 2 times a day everyday, there's a program for you too. The key is find a program you like, because you're more likely to continue with it.
Here are some videos that cover the basics of preparation and race transitions.