I can't tell you how many times I've started a workout program, whether at the gym or in my own home, that I ended up not finishing. I would always find excuses as to why I just shouldn't do it, why I was too tired, or my favorite "I wasn't seeing any results anyway so what's the point." The most critical error I made all those times was the fact that I didn't choose a program that was suited to me. I picked the programs or the workout circuits at the gym that other people said were the best to help you lose weight or to get lean muscle. But the problem with that was that I HATED all of the workouts. There was too much cardio, or the exercises were too repetitive, or not fun enough. I wasn't invested enough to keep going and I hadn't developed the habit of working out yet so I'd quit over and over again.
The most important advice I can give you is to pick a program you think you will LIKE! Don't pick the program your friend is doing, or the one that everyone says is the best. You need to find the program that you will finish and is suited to your fitness level at that time. Take me for example; Ryan tried to get me to do Insanity with him a year or so ago. He told me it was the best way to lose fat and I'd get fantastic results. So I thought to myself, why not? Well let me tell you...it was AWFUL! I stopped after the warm up!!! It was the craziest cardio I had ever experienced in the first 3 mins of the workout. I can't begin to tell you how angry I was at Ryan for "making me" do that program. The point is, if you don't like cardio, don't choose a program that is 100% cardio based. I ended up sticking to 21 Day Fix and Hammer and Chisel because they were primarily weight based and I enjoyed them (as much as I can enjoy working out).
If you have bad knees, stick to a program that is low impact. If you want to improve your flexibility, choose something like yoga or pilates. Don't choose a program because someone told you to. That is the easiest way to set yourself up to fail.
You need to start your health and fitness journey right, by choosing the best possible workout regime to get you on the right path. I chose to start with an at home workout and nutrition plan, knowing it was the most likely way for me to finish the entire program and because I liked the routines and found the nutrition easy to follow, I kept going. Now that I have developed a habit of working out, I've switched to Insanity Max 30. This is a program I am not particularily fond of, but I am able to push through the workouts because I now have a better mindset and appreciation of the benefits of working out. If you would have told me I had to do that program even 4 months ago, I would have quit. It's all about preparing yourself to succeed! If you pick the right program you are already 1/2 way there. You will succeed because you chose a program you will STICK to. As long as you believe in yourself, have people who motivate you, and love the workouts you "have" to do, you have already won!