Once you've picked your activity, decide how often a week you want to do it. At least three times is the rule. Then pick where those workout times are going to fit into your schedule. You have to make room for them. Will it be thirty minutes on a lunch break? Will it be in the evening after work? In the morning before classes? While the kids take a nap? While Dad is giving them all their baths? Whatever works well for you is when you should do it. Fitness is fairly flexible. So long as the gym is open or it isn't too dark and dangerous at the park for a jog, etc., you can go when it feels good to you to go.
Okay, here's the real clincher. Once you've got it planned, start it and keep it up consistently for two weeks. Short of a seriously necessary interruption, don't let anything stop you from doing it that can be avoided. Follow through with your plan for two weeks. After two weeks you will be in significantly better shape than when you started.
You will have gotten used to the physical load. You will have gotten over being sore after every time you do it. You will have gotten used to the new weekly routine. You will have gotten used to having this activity in your life. If you make it to your two-week goal, doing it will have gotten significantly easier. And insisting that you do it when you don't want to will be easier to, because you've been practicing it for two weeks
If you do this, you are much more likely to continue the activity for life. And once you see what a difference the two-week commitment makes for you, it will be easier to get your fitness life going again if you ever get off track. Just tell yourself, "Remember last time? In just two weeks it was much easier. Come on, self-I can force you to do it for two weeks and then it will be easier to stick with it after that."
In another two weeks, you can add more if you are interested. Add a strength-training workout or a cardio workout to compliment what you're already doing. Maybe lengthen the amount of time you work out or add another day or two a week. Focus on this as something that you need in order to take good care of yourself. Just like you do other things necessary for your health, like eat and drink and brush your teeth and sleep. I know, some people don't do that stuff well, either. Enjoy your life and take care of it. A big part of the meaning of life is having fun and feeling good, and health and fitness are a big key to succeeding in that.