Many people are happy not to have to do physical work but instead to sit in an air conditioned office and leave work not completely exhausted. Unfortunately as much as a sedentary job may be good in terms of not forcing you to be outside in extreme weather and physically exhausting you each day until you move on to a different job or retire, it does have a negative impact on your body, so unless you want to get up from your desk one day only to find out you can't straighten your back properly, you need to take some precautions. And the sooner you do that, the better.
One of the main things that happen to people who work in a sitting position is weight gain. You don't get enough movement, you eat at your computer and before you know it those couture dresses or fancy suits are tight in the waist. It's not only bad for your self image, but most importantly for your joints, back and heart. Overweight people also are more prone to diabetes, so you have to watch your weight not for vanity but for health reasons.
Make sure you eat right and that you pay attention. Take a ten minute break from your computer and have a grilled chicken salad stuffed in pita bread instead of a burger with fries, grab pastrami on rye rather than pizza and try to always have fruit or veggies with lunch or as a snack. If you concentrate on eating you will eat less and you will stay full longer than if you take a bite in between typing new paragraphs, so be attentive to your food.
Eating better is a good change, but it won't work on its own. To keep your weight in check, but also to strengthen your body and not let your joints to get stiff, you need to exercise on a regular basis. Find a fitness regimen that you can stick to, something that not only helps your body but also lets you forget about work for awhile. Whether it's yoga, tennis, spinning or swimming, make sure that you do it at least once a week and that you do it right, with a proper warm up and technique, because otherwise you can do more harm than good.
Aside from regular exercise outside of work, you should also start stretching in the office. It doesn't have to be a full routine every hour on the hour, but you should get away from your desk several times a day and work your whole body with some twists, bends and sit ups, so you don't get any tension or pressure in any part of your body and you can move freely after getting up.
Even the best exercise won't help you much if your position is wrong and your office furniture doesn't support you. Make sure the desk and the chair are set to your height, that you have proper support of your back, that your feet reach the ground, that your monitor is far enough and at the right level. Only when your work station is set up right can you be sure that you're stretching, exercise and proper nutrition will keep your body healthy for a long time after you get away from your desk.