Sitting in front of a computer is good, especially if you make money from it. Alas, too much sitting is not good for a human being.
Make sure your back is straight up, and your arms bent at about 90 degrees. Your arms should feel relaxed. The upper middle of your computer screen should be at the height of your eyes, so that you can look straight forward without having to bend your neck.
Avoid crossing your legs as it is not good for the lower back. Both feet should be on the ground.
Take a stroll and stretch each 15 minutes or so. A couple of good stretches and exercises:
If you feel moderate or strong pain in the neck or in the back you should have a massage and having a good therapist is equally important as sitting in a good posture. Having one back massage a week would be ideal, and if you cannot afford it you can ask a friend to do it.
So that's all. Sit properly, walk, stretch, have a massage and your body will be grateful! You will have more energy and feel better.