A very easy schedule and a nice, gradual way to get into the habit without losing motivation is to have an easy, relaxed workout every morning, short bursts of cardio a couple of times a day and some more focused workouts a few times a week (my focused stuff at the moment is back pain stretching and taking cardio up one notch).
I think the best way to start is to make a list of priorities, as you can't work on all your goals at once. My ultimate goal is to gain weight and have some nice, natural looking muscles, but I am not working on that at all right now, because my priorities at the moment are soothing my back pain (an obstacle for working out), making myself more flexible (I have joints made out of plaster and they cramp up every time I make an uncommon move) and just getting generally in a good enough shape that will allow me more intense future workouts without hurting myself and without having to stop in the middle because I am not strong enough (pulmonary capacity, breathing techniques, resistance to pain, endurance).
You might therefore want to start with stretching (this can also be yoga) every morning and maybe doing a bit of jogging (just long enough to get your blood pumping to wake you up) or some other light cardio. A jog around the block is plenty enough. Push-ups and situps are good, too, if you can handle them.
After work/school, you would do about fifteen minutes of cardio every day. Trust me, you don't want to do more at first because your muscles might hurt a bit too much the next day, which might make you want to skip workouts, which will not be helpful. And then, a couple of times per week, you can focus on your long term goal. If that is weight loss, you can do some longer cardio sessions, but make sure you don't tire yourself out too much. If your main goal is getting flexible, you might want to do power yoga (regular yoga won't get your heart rate fast enough for keeping in shape). If your main goal is toning up, do body weight exercises. If you want muscle, do weight training, body weight exercises or targeted workouts for your abs, arms, chest, butt or legs.
You don't need to go to the gym for any of this. Having to go to the gym might introduce a time constraint, especially for busy people. That might demotivate you. Then again, some people might have better motivation going to the gym and seeing other people looking good and keeping at it. This depends on your personality and level of motivation.
To make a long story short, it is easier at first to do brief workouts every day several times a day than to do three serious workouts per week. If your pulmonary capacity, your blood pressure, your flexibility or your strength is below average, you might not be able to see workouts through to the end, and that will mess up your motivation. This is what happened to me the very first day – I just couldn't keep going and had to stop halfway through. Stick with ten minute workouts, fifteen at the most. And stick with the casual stuff for now. Even some of the casual stuff is too much for me...
Break it all down into smaller, more immediate, manageable goals and concentrate on that, all the while keeping a wider perspective and remembering why you are doing this and what your ultimate goal is.
And please, do your warmups. They only take a couple of minutes. You'll be glad you did!