Yes, you read that right: "I'll start exercising when I get in better shape." I hear it a lot, and I still don't get it. I'll try to be kind, but what are these people talking about?! Unless you're morbidly overweight and need medical supervision to lose weight and get out of bed and walk a little, I think this is a protective mechanism, i.e. an excuse!
Sometimes I hear this from people who come to a class, and while they're challenged and a bit outside their comfort zones, they do well! The finish class, still breathing, limbs intact. They did it! But then they make excuses why they can't do it again because they found it challenging. Uh, you just did it! You should be happy with yourself, instead of beating yourself up that it was hard and you are a bit sore the next day. That is called results!
Or there are those who come to a class, do it, do it well but because they are sore afterwards, they throw in the towel and say they'll come back in a few months. Hate to say it, but you'll be sore again in a couple of months. To see progress in your exercise routine, you need to overload the muscles to gain strength and endurance. It's like driving a car up a steep hill. If you don't push it a bit harder, you won't make it over the hill, but just roll back down the hill to where you started.
Or maybe they are active, but do some sort of activity that is completely different from what is offered in the studio. Say they run, but resistance training is a real but kicker for them. No amount of running is going to make resistance training easier. (The opposite is true, though; resistance training can vastly improve your running). Cross training can help with performance, cut down on repetitive stress injuries, and relieve boredom.
So, where to start? I suggest talking to a trainer, like me! ;-) Or consider my classes. I keep them small so I can provide personal attention, modifications and/or alternative exercises. Or review your gym's class schedule to see what is appropriate for beginners to athletes. You'll be amazed at what you can do! Just do it!